Asthma outcomes: Biomarkers

doi:10.1016/j.jaci.2011.12.979

J Allergy Clin Immunol. Author manuscript; available in PMC 2012 September 1.

Published in final edited form as:

J Allergy Clin Immunol. 2012 March; 129(3 Suppl): S9–23.

doi: 10.1016/j.jaci.2011.12.979

PMCID: PMC3390196

NIHMSID: NIHMS387444

Asthma outcomes: Biomarkers

Stanley J. Szefler, MD, coprimary author,^a Sally Wenzel, MD, coprimary author,^b Robert Brown, MD, MPH,^c Serpil C. Erzurum, MD,^d John V. Fahy, MD, MSc,^e Robert G. Hamilton, PhD,^c John F. Hunt, MD,^f Hirohito Kita, MD,^g Andrew H. Liu, MD,^a Reynold A. Panettieri, Jr, MD,^h Robert P. Schleimer, PhD,ⁱ and Michael Minnicozzi, PhD^j

^aNational Jewish Health, Denver

^bUniversity of Pittsburgh

^cJohns Hopkins University, Baltimore

^dCleveland Clinic Foundation

^eUniversity of California, San Francisco

^fUniversity of Virginia, Charlottesville

^gMayo Clinic Rochester

^hUniversity of Pennsylvania, Philadelphia

ⁱNorthwestern University, Chicago

^jNational Institute of Allergy and Infectious Diseases, Bethesda

Corresponding author: Michael Minnicozzi, PhD, Division of Allergy, Immunology, and Transplantation (DAIT), National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH), 6610 Rockledge Drive, Bethesda, MD 20892. Email: minnicozzim/at/niaid.nih.gov

The publisher's final edited version of this article is available at J Allergy Clin Immunol

See other articles in PMC that cite the published article.

Abstract

Background

Measurement of biomarkers has been incorporated within clinical research studies of asthma to characterize the population and associate the disease with environmental and therapeutic effects.

Objective

National Institutes of Health institutes and federal agencies convened an expert group to propose which biomarkers should be assessed as standardized asthma outcomes in future clinical research studies.

Methods

We conducted a comprehensive search of the literature to identify studies that developed and/or tested asthma biomarkers. We identified biomarkers relevant to the underlying disease process progression and response to treatment. We classified the biomarkers as either core (required in future studies), supplemental (used according to study aims and standardized), or emerging (requiring validation and standardization). This work was discussed at an National Institutes of Health–organized workshop convened in March 2010 and finalized in September 2011.

Results

Ten measures were identified; only 1, multiallergen screening to define atopy, is recommended as a core asthma outcome. Complete blood counts to measure total eosinophils, fractional exhaled nitric oxide (Feno), sputum eosinophils, urinary leukotrienes, and total and allergen-specific IgE are recommended as supplemental measures. Measurement of sputum polymorphonuclear leukocytes and other analytes, cortisol measures, airway imaging, breath markers, and system-wide studies (eg, genomics, proteomics) are considered as emerging outcome measures.

Conclusion

The working group participants propose the use of multiallergen screening in all asthma clinical trials to characterize study populations with respect to atopic status. Blood, sputum, and urine specimens should be stored in biobanks, and standard procedures should be developed to harmonize sample collection for clinical trial biorepositories.

Keywords: Multiallergen screen, fractional exhaled nitric oxide, sputum eosinophils, total eosinophils, IgE, urinary leukotriene E₄

Asthma clinical research lacks adequate outcomes standardization. As a result, our ability to examine and compare outcomes across clinical trials and clinical studies, interpret evaluations of new and available therapeutic modalities for this disease at a scale larger than a single trial, and pool data for observational studies (eg, genetics, genomics, pharmacoeconomics) is impaired.⁴ Several National Institutes of Health (NIH) institutes that support asthma research (the National Heart, Lung, and Blood Institute; the National Institute of Allergy and Infectious Diseases; the National Institute of Environmental Health Sciences; and the Eunice Kennedy Shriver National Institute of Child Health and Human Development), as well as the Agency for Healthcare Research and Quality, have agreed to an effort for outcomes standardization. This effort aims at (1) establishing standard definitions and data collection methodologies for validated outcome measures in asthma clinical research with the goal of enabling comparisons across asthma research studies and clinical trials and (2) identifying promising outcome measures for asthma clinical research that require further development. In the context of this effort, 7 expert subcommittees were established to propose and define outcomes under 3 categories—core, supplemental, and emerging:

Core outcomes are identified as a selective set of asthma outcomes to be considered by participating NIH institutes and other federal agencies as requirements for institute/agency-initiated funding of clinical trials and large observational studies in asthma.
Supplemental outcomes are asthma outcomes for which standard definitions can or have been developed, methods for measurement can be specified, and validity has been proved but whose inclusion in funded clinical asthma research will be optional.
Emerging outcomes are asthma outcomes that have the potential to (1) expand and/or improve current aspects of disease monitoring and (2) improve translation of basic and animal model–based asthma research into clinical research. Emerging outcomes may be new or may have been previously used in asthma clinical research, but they are not yet standardized and require further development and validation.

Each subcommittee used the recently published American Thoracic Society (ATS)/European Respiratory Society (ERS) Statement: Asthma Control and Exacerbations—Standardizing Endpoints for Clinical Asthma Trials and Clinical Practice⁵ (hereafter referred to as the ATS/ERS Statement) as a starting point and updated, expanded, or modified its recommendations as the subcommittee deemed appropriate. Each subcommittee produced a report that was discussed, modified, and adopted by the Asthma Outcomes Workshop that took place in Bethesda, Md, on March 15 and 16, 2010. The reports were revised accordingly and finalized in September 2011. The recommendations of the workshop participants in regard to asthma biomarkers are presented in this article and summarized in Tables I, ,IIII^1–3, and III.

TABLE I

Recommendations for classifying asthma biomarker outcome measures for NIH-initiated clinical research for adults and children

TABLE II

Methods for measuring and reporting core and supplemental biomarker outcomes

The measurement of biomarkers has been incorporated within clinical studies of asthma to characterize the participant population and to try to associate the disease process with environmental effects and therapeutic interventions.

This summary highlights the current knowledge regarding biomarkers deemed applicable as core measures, specifically the multiallergen screen to define atopy, and supplemental biomarkers, which include measurements of sputum eosinophils, IgE, complete blood count (CBC), fractional exhaled nitric oxide (Feno), and urinary leukotrienes. In addition, several biomarkers are considered emerging measures, such as imaging and cortisol. Future studies will be required to further validate and characterize their roles as outcome measures. We also discuss the issue of proper storage of biologic samples to allow future analyses.

The conclusions of the Asthma Biomarkers Subcommittee extend those included in the ATS/ERS Statement by considering additional publications available since that report and by focusing our comments on the role of biomarkers in clinical research.⁵ There are no essential disagreements with the ATS/ERS Statement.

REVIEW OF SPECIFIC CORE AND SUPPLEMENTAL BIOMARKER OUTCOMES

Total and allergen-specific IgE

Summary

Atopic status is an important phenotype and should be documented in clinical research studies to permit adequate interpretation of study findings. The presence of allergen-specific IgE is a biomarker for atopic asthma.
The multiallergen screen is a single semiquantitative serologic measure of IgE against major allergens. It is considered a core biomarker that permits characterization of the atopic status of a study population in prospective clinical trials and observational studies. It characterizes an individual as atopic but does not specify as to which allergen(s) a person is sensitized.
Quantitative serologic measures of individual allergen-specific IgE antibodies offer information on specific allergen sensitivities and are considered supplemental biomarkers for study population characterization, assessment of efficacy and effectiveness outcomes in intervention studies, and in observational studies, as deemed appropriate by study design.
Measurement of allergen-specific IgE by skin prick test, although widely used in the clinical setting, is considered an emerging biomarker for research because of the variability of the test’s performance.
Total serum IgE has been associated with asthma and is considered a supplemental measure for study population characterization, as well as an outcome for intervention and observational studies, as deemed appropriate by study design.

Medical and scientific value

The quantity of total IgE and presence of allergen-specific IgE antibody in serum are both important biomarkers for defining the phenotype of a patient who presents with asthma symptoms.⁶ The titers of allergen-specific IgE in serum also may be useful in predicting persistent wheeze and in targeting allergen specificities for allergen avoidance management. Detection of local IgE antibody in the skin and extracts of tissue may aid in adjudicating negative in vivo and serologic measures of IgE antibody despite clinical evidence of atopic asthma.^6–11

Total serum IgE has been associated with asthma.^12–14 Serum IgE levels are highly age-dependent: Atopic infants have an earlier and steeper rise in serum IgE levels than age-matched nonatopic controls.^15,16 Total serum IgE reaches adult levels by age 10 to 15 years and gradually declines from the second decade of life. However, a bigger problem is the considerable overlap in IgE levels between atopic and nonatopic populations, which reduces its utility in identifying atopy.

Allergen-specific IgE defines an individual as having atopic asthma.^6,17,18 It confirms sensitization in support of a clinical history-based diagnosis and aids in identifying allergen triggers. The probability of wheeze and reduced lung function increases with increasing specific IgE levels in serum.¹⁸ Summed levels of mite-, cat-, and dog-specific IgE in 3-year-old children were associated with a 1.33-fold increase in the probability of wheeze by age 5 (95% CI, 1.21–1.47; P <.0001) per logarithmic unit increase in IgE antibody, corresponding to an odds ratio of 3.1 at 10 and 4.25 at 30 kUa/L (kilounits of antibody per liter of serum). In contrast, current wheeze was not associated with the size of skin test wheal.¹⁸

The use of the multiallergen screen for aeroallergens (Phadiatop) in combination with the food allergen mix (fx5) has been more effective than individual allergen-specific IgE measurements in characterizing the atopic status of children.^19,20 When used together to evaluate 4-year-old children, these 2 screening tests exhibited a 97.4% positive predictive value for any suspected allergic disease (asthma, rhinitis, atopic eczema/dermatitis syndrome, and food allergy).¹⁹

Definition and methodology for measurement

Quantitative measures of total serum IgE can be equivalently obtained from any of the US Food and Drug Administration (FDA)–cleared immunoassays from HYCOR (EIA), Phadia (now Thermo Fisher Scientific) (ImmunoCAP), or Siemens (IMMULITE, ADVIA Centaur, and Nephelometer) that are used in clinical laboratories throughout North America.²¹ Total serum IgE data from the College of American Pathologists’ external proficiency studies display excellent intermethod agreement (coefficients of variation less than 15%) and comparable performance (precision, linearity, analytic sensitivity to 2 IU/mL) for all 5 assays.²² Thus any of the above total serum IgE assay methods can be used without concerns for comparability. In term of units, 1 IU is equivalent to 2.4 ng of IgE.

Allergen-specific IgE can be measured in serum by 1 of 3 immunoassays or in the skin by using any 1 of the many technical variants of prick and intradermal skin test methods. The skin prick test method has greater diagnostic value, but the lack of full standardization reduces its value in clinical research, especially when cross-study comparisons are considered.²³ Serologic measurements of allergen-specific IgE can be performed using the HY-COR (HYTEC), Phadia (ImmunoCAP), and Siemens (IMMULITE) autoanalyzers. All 3 assays display excellent precision, reproducibility, linearity, and equivalent analytic sensitivity (0.1 kUa/L).²² However, the 3 assays measure different levels of IgE antibody to any given allergen specificity. Because the results generated with the 3 assay methods are qualitatively equivalent but not quantitatively identical, the subcommittee recommends the Phadiatop and fx5 (ImmunoCAP) as the IgE antibody analyses for asthma clinical trials. This is because the performance of the Phadiatop multiallergen screen has been the most well documented of the 3 available multiallergen screens.

One version of the IgE antibody assay is the multiallergen screen.^19,20,24,25 It is a single analytic measurement, the adult (≥15 years old) version of which simultaneously detects specific IgE antibody to any of 10 aeroallergens that cross allergen groups and includes aeroallergens in the dust mite, pet epidermal, grass, tree and weed pollen, and mold families. Phadia’s version of the multiallergen screen, the Phadiatop, generates both a dichotomous positive/negative value and a semiquantitative (kilounits of allergen per liter) estimate of relative positivity. Although the presence of IgE antibody indicates a state of atopy, the precise IgE antibody specificity of a positive result is not defined.^19,20,24,25 The test is proposed as a core biomarker for characterization of the atopic status of study populations for prospective clinical trials and observation studies. The assay’s analytic sensitivity was reduced from 0.35 to 0.1 kUa/L in March 2008; however, performance studies to date have continued to use a 0.35 kUa/L positive cut point criterion to define the presence of atopy. The clinical relevance of results in the 0.1 to 0.35 kUa/L range is currently undetermined.
In children with asthma (<15 years of age), the fx5 food allergen mix should be added to the adult Phadiatop because food allergy is more common in this age group and needs to be included when assessing for atopy. The fx5 is a single test that simultaneously detects IgE to any of 6 foods (chicken egg, cow’s milk, peanut, soybean, codfish, and wheat) that are the principal sensitizing food allergens for children. This dual test strategy has been selected over the Phadiatop Infant,^19,20,26,27 which also includes food allergens because the paired adult Phadiatop-fx5 combination is a more comprehensive assessment of atopy; performance of the Phadiatop Infant has not been studied in a US population; and unlike the Phadiatop Infant, the adult Phadiatop and fx5 are both FDA cleared.
Clinics can order specific IgE antibody tests to more than 200 individual allergen specificities, each denoted with a letter-number code (eg, D1 corresponds to Dermatophagoides pteronyssinus [dust mite]).¹⁶ Individually performed specific IgE tests have been classified as supplemental biomarkers because the participant’s clinical history is needed to identify the target allergens for testing, and more than 1 specific IgE antibody test is generally needed to characterize a particular participant’s sensitivities.

In vivo measurements of allergen-specific IgE in the skin are an alternative to serologic assays. Because we could not identify a single generally accepted technique, we have classified skin testing as an emerging biomarker in this report. The following issues need resolution before skin testing can be reclassified as a supplemental biomarker²⁸: (1) allergen extract potency, stability, concentration, levels of irritant, and other contaminants need to be more uniform; (2) the technique must be standardized with respect to use of skin prick versus intradermal methods, choice among various skin test devices, number of skin tests performed, reporting scale, use of wheal or erythema as outcome, comparison with saline or histamine controls, skin test spacing, and extent of needle penetration; (3) grading scheme and interpretation strategy need to be clearly defined; and (4) patient variables, such as dermographism and interfering premedications, need to be considered. Finally, the analytic sensitivity of skin testing is unknown.

Repeatability

The results obtained with total and allergen-specific IgE autoanalyzers are highly reproducible, with intra-method interlaboratory coefficients of variation less than 15% based on multilaboratory proficiency survey data.²² However, a number of nonanalytic factors can alter total and allergen-specific IgE levels over time, including age, seasonal allergen exposure, and parasitic infections. Although originally a concern, pretreatment with anti-human IgE (eg, omalizumab) does not significantly alter total or specific IgE measurements in the ImmunoCAP system.²⁹

Skin testing results, in contrast, show inherent variations that are influenced by extract quality, technique, device, experience, and methods of interpretation.²⁸

Responsiveness

Total and allergen-specific IgE levels might be affected by pharmacotherapy using standard asthma medications. Corticosteroid administration has been reported to induce IgE synthesis but also to decrease serum IgE levels, especially in allergic bronchopulmonary aspergillosis (ABPA).^30,31 Response to treatment with omalizumab, which increases total IgE but reduces free IgE levels, can be accurately monitored by Immuno-CAP when omalizumab is present in serum.²⁹ Certain medications can interfere with skin test performance.

Validity

Total serum IgE level is an insensitive indicator of asthma outcome. Allergen-specific IgE antibody levels better reflect the extent of sensitization (atopic state) and might be useful in predicting a predisposition for prolonged wheeze.^17,18 However, as mentioned above, allergen-specific IgE, as measured by serology or skin tests, is an insensitive indicator for assessing clinical asthma outcomes or for predicting future manifestations of asthma control.

Associations

An association between total serum IgE levels and asthma has been reported.^12–14,32 However, asthma heritability is only partly related to the familial aggregation of total serum IgE. In the absence of a parent with asthma, asthma prevalence was significantly higher in children when both parents had total serum IgE levels in the highest tertile.¹² Children with asthma also have higher total IgE levels than predicted by parental IgE levels alone. It has been speculated that total IgE changes in patients with asthma are indirect measures of airway inflammation. However, in most studies this association is weak.

Ina 2001 study involving an American population of children and adolescents (aged 6–18 years) with a history of rhinitis and an asthma prevalence of 59%, the Phadiatop displayed a diagnostic sensitivity of 98% against skin prick test reference and 83.2% against a clinical history reference using a 0.35 kUa/L positive cutpoint.²⁴ In a 2009 epidemiology study of European children (median age, 2.7 years), in which 122 children were classified as atopic (70%) or nonatopic (30%) by a combined clinical evaluation, skin prick test, and itemized serum IgE antibody analysis, the Phadiatop Infant (which includes principal food allergens) displayed positive and negative predictive values of 95% (95% CI, 89% to 99%) and 94% (95% CI, 89% to 99%), respectively, using a 0.35 kUa/L positive cutpoint.²⁷ The combined use of the Phadiatop and fx5 has been shown to effectively predict the atopic state (>0.35 kUa/L = IgE antibody positivity) by age 4 years. The combined degree of positivity correlated with the severity of recurrent wheeze and limited peak flows in a pediatric asthmatic population.^19,20

Increased probability of wheeze and reduced lung function are associated with increasing specific IgE antibody levels as measured in serum but not by skin tests.¹⁸ Increasing summed quantities of mite-, cat-, and dog-specific IgE at age 3 years significantly increased the risk of persistent wheeze by age 5 years.¹⁸

Practicality and risk

Total and allergen-specific IgE assays are standardized and performed in federally licensed clinical immunology laboratories. Their practicality is enhanced by the fact that serum can be retrieved from long-term storage repositories. Skin testing may be practical in the clinical setting (results immediately available) but less so in clinical research, given that it requires a significant time commitment from both study participants and research staff. Although very small, the risk of adverse reactions is higher with skin testing than with IgE antibody serology. In comparative studies involving aeroallergens, intradermal skin test results add little to the diagnostic evaluation obtained with skin prick test results.²³

Demographic considerations

Age, gender, and race may affect the levels of total and allergen-specific IgE antibody levels as measured in serum or by skin test.^{12–15,32,33}

Priority for NIH-initiated clinical research

The atopic status of patients with asthma can be a determinant of management choices, as well as of the response to therapy and prognosis. Therefore, it is important that the atopic status of a study population is reported. The results of the multiallergen screen (Phadiatop), which is a single semiquantitative serologic measure of allergen-specific IgE across major aeroallergens, and in children younger than 15 years, the addition of the fx5 (covering food allergens) are considered core biomarkers that permit characterization of the atopic status of a study population in prospective clinical trial and observational studies. Quantitative serologic measures of total and individual allergen-specificities of IgE antibody are considered supplemental biomarker outcomes for study population characterization, for assessment of efficacy and effectiveness in intervention studies, and for use in observational studies. Measurement of allergen-specific IgE by skin test is considered an emerging biomarker because of lack of standardization and the many factors that affect the test’s performance and interpretation of results.

Exhaled nitric oxide

Summary

Feno measured at a constant flow rate is a simple, safe, and reproducible biomarker for use in asthma clinical trials.
Although Feno values overlap among healthy, atopic, and asthmatic cohorts, changes in Feno values over time in individuals who have asthma are relevant to clinical research studies that seek to measure effects of interventions on airway inflammation, in particular the effects of anti-inflammatory (eg, corticosteroid) therapies.
Feno levels of less than 25 ppb generally indicate lower likelihood for eosinophilic inflammation and responsiveness to corticosteroids. However, Feno cannot be used interchangeably with sputum eosinophilia as an outcome measure, given that eosinophilic inflammation and Feno levels do not always respond identically to treatment.
Feno is recommended as a supplemental outcome in clinical trials that seek to evaluate effects of interventions on airway disease and/or to characterize corticosteroid-responsive phenotypes of asthma.

Definition and methodology for measurement

Definition

Measurement of Feno is a quantitative measure of airway nitric oxide (NO), a gaseous mediator produced endogenously in cells by NO synthases. Exhaled NO is commonly regarded as an indirect marker for airway inflammation. The joint ATS/ERS guidelines for the measurement of Feno are the current standard.^1–3

Methodology

Exhaled NO is reported in parts per billion in exhaled breath. Measurement methods are well described in the ATS/ERS guidelines.^1–3 Online measures refers to the study participant exhaling directly into the instrument. Offline measures refers to the study participant exhaling from total lung capacity into a nonpermeable gas collection bag that is subsequently sampled for measurement of NO. Either technique is valid and accurate for comparison of NO among populations, but offline measures are not interchangeable with those measured online due to differences in the methodology mostly related to flow dependence. Flow rate of exhalation affects the level of Feno, with faster flow rates resulting in lower Feno levels and slower flow rates resulting in higher Feno levels. An exhalation flow rate of 0.35 L/sec is recommended for offline measures, whereas a flow rate of 0.05 L/sec is recommended for online measures, and the exhalation should be sufficient to obtain an NO plateau for at least 3 seconds. Online Feno measurement, as opposed to offline, is suggested for most clinical trials of asthma as it is more suited to standardization at multiple sites because FDA-approved equipment for online measurement is readily available in most pulmonary function laboratories. However, offline measures of NO may be useful in field studies with remote sample collection or when specific experimental study design calls for investigational assessment of total lung volume NO.

Medical and scientific value

NO is generally accepted as a marker of airway inflammation. Individuals who have asthma have been shown to exhale high levels of NO, which decreased in response to corticosteroids. The testing is noninvasive, easy to perform in children and patients with severe airflow obstruction, and has no risk to patients.

Range of values

Normal ranges

Several publications have reported reference values for Feno in adults^34–38 and children.^39–44 In general, the upper value of normal for online measure of Feno is 25 ppb.

Distribution of NO in individuals with and without asthma

Mean Feno levels in populations without asthma and populations with asthma overlap significantly, but the distribution of Feno in populations with asthma is generally higher than that in a population without asthma. In patients with stable well-controlled asthma, Feno values have been reported to range from 22 to 44 ppb.⁴⁵

Numeric transformation

Feno values are provided directly from NO analyzers in parts per billion.

Repeatability

The coefficient of variation of Feno of healthy individuals is approximately 10% (about 4 ppb),^46–48 whereas the variation in individuals with asthma ranges from 20% to 40%.^46,49

Variation in results across sites

Using the ATS/ERS standard methods for online measure of exhaled NO will minimize variation across sites.^1,2

Responsiveness

Many studies describe the correlation of Feno and eosinophilic airway inflammation. Feno is related to eosinophil numbers in bronchoalveolar lavage fluid,⁵⁰ bronchial biopsies,⁵¹ and induced sputum.^52–55 In general, however, a low Feno is more likely to exclude airway eosinophilia than a high Feno is likely to predict it. Specifically, in symptomatic adults who have asthma with Feno less than 25 ppb, eosinophilic airway inflammation is unlikely. Feno greater than 47 ppb suggests eosinophilic inflammation and corticosteroid-responsive asthma, but persistently high Feno in a patient with ongoing asthma symptoms may occur despite adequate anti-inflammatory treatment.⁵⁶ A clinically important decrease of Feno is defined as a change of 20% for values over 50 ppb (or a change of 10 ppb for values lower than 50 ppb) that occurs 2 to 6 weeks after initiation of corticosteroid therapy.^3,40,57,58

Validity

Feno values are generally higher in individuals with asthma than in healthy controls and reflect lower airway inflammation. However, elevated levels may commonly be seen in atopic individuals without asthma.

Associations

Strength and direction of associations

While studies identify the association of Feno and eosinophilic airway inflammation,^52–54,59 the sensitivity and specificity of Feno for sputum eosinophila are only approximately 70%, and the relationship between Feno and eosinophilia may occur independently of asthma control.⁶⁰ The relationship between Feno and eosinophilia is not exact, in part because sputum eosinophilia is never found in healthy airways, but NO is present in health and the distribution of values is skewed rightward in the normal range.

Additional information provided by this variable

Feno does not duplicate other outcome measures of inflammation, particularly sputum eosinophilia. For example, corticosteroid therapy reduces Feno values, but anti–IL-5 and anti-IgE therapy for asthma reduce sputum eosinophilia without affecting Feno.⁶¹ Feno is also related to atopy as measured by skin prick test positivity,^62,63 such that well-controlled individuals with asthma and positive allergen skin prick tests have increased Feno levels.^64,65

Practicality and risk

The test is easy to perform, quick, simple, and well tolerated by participants. Little to no variation in outcomes has been observed across sites using FDA-approved devices under ATS/ERS guidelines. The cost of purchasing and maintaining equipment may be prohibitive for some studies. As less expensive and portable handheld devices are developed, this may be less of a limitation; however, these devices need further evaluation as clinical research tools. No safety issues have been identified.

Demographic considerations

Feno levels are generally lower in children than in adults, whether they have asthma or not^{39,40,42–44}; for example, mean Feno levels in children aged 12 years or less are approximately 5 ppb less than those of adults. Men’s Feno values are approximately 25% higher than those of women.^37,66,67 Body mass index is inversely associated with Feno in people with asthma, and weight loss may affect levels.⁶⁸

Priority for NIH-initiated clinical research

Feno is recommended as a supplemental outcome for the characterization of study populations, for prospective clinical trials, and for observational studies. Measurements are simple to make, with no risk to participants, and are useful in the assessment of airway inflammation. Feno and sputum eosinophilia are not duplicative outcome measures (ie, reduction in the levels of these biomarkers does not occur in parallel), even though low sputum eosinophilia and low Feno are strongly linked. Feno values overlap between healthy, atopic, and asthmatic individuals and do not generally reflect severity of disease. Additionally, the equipment is expensive, and further information on its biologic relevance is needed. At this point, within-individual changes in Feno values over time may be most relevant to clinical research studies that seek to measure effects of interventions on airway inflammation, in particular effects of corticosteroid-like therapies. Thus although not recommended as a core measure at this time, as data continue to accumulate regarding the relevance of Feno to clinical outcomes, it is possible that this may become a core biomarker in the future.

Future directions or research questions

The inflammatory factor(s) that promote increased Feno remain unclear, as does the reason for the disconnect between the presence of eosinophilic inflammation and Feno. To optimize the use of this biomarker, future studies that determine the relationship of Feno to asthma control, asthma phenotypes, and airway remodeling in relation to corticosteroid use are needed.

Sputum eosinophils

Summary

Analysis of eosinophil counts in induced sputum identifies patients who have eosinophilic and noneosinophilic phenotypes of asthma. These inflammatory phenotypes can predict response to treatment.

Definition and methodology for measurement

Total and differential counts in induced sputum samples will yield both the eosinophil percentage and the total eosinophil number per milliliter of sputum, which can both be reported. However, the eosinophil percentage outcome is preferred because the percentage transformation controls for the effects of saliva in the sample, which can dilute the concentration of eosinophils.

Two main methods are in use. In the United States, most NIH-funded studies and networks use the whole expectorate method,^69,70 whereas Canadian and European investigators tend to use the sputum plug method.⁷¹ The main elements of these 2 methods are described below:

The whole expectorate method entails the following steps: pretreatment with 4 puffs of albuterol; inhalation of 3% saline for 12 minutes (the duration of sputum induction with this method is standardized at 12 minutes, based on data showing this time length to be optimal for the collection of airway secretions⁷⁰); at a minimum of 2-minute intervals, the study participant spits saliva into one cup before coughing sputum into another; peak flow or FEV₁ is monitored at 2-minute intervals. The whole expectorate is mixed with an equal volume of 10% dithiothreitol and the sample homogenized in a shaking water bath for 15 minutes with intermittent aspiration of the samples by transfer pipette. An aliquot of this sputum is cytocentrifuged to generate cytology slides that can be stained to allow identification of leukocytes. The homogenized sputum also can be centrifuged to yield aliquots of sputum supernatant for measures of inflammatory proteins in the fluid phase.⁶⁹
The sputum plug method consists of the following steps: pretreatment with albuterol; inhalation of 0.9%, 3%, 5%, and 7% saline at 5- to 10-minute intervals. Sputum is expectorated into a container from which the mucus plugs are selected using a wooden spatula.⁷¹ The mucus plug material is processed in dithiothreitol using methods similar to the whole expectorate method, described above.

Few studies have compared data generated by these 2 methods, but available data suggest that the 2 methods yield similar data for sputum eosinophil percentage. Until further comparative studies are available, we recommend that the whole expectorate method be used for NIH-funded studies because that is the current practice.

Medical and scientific value

Airway eosinophilia is a well-defined inflammatory characteristic of a phenotype of asthma likely orchestrated by T_H2 cytokines and known to be responsive to corticosteroid treatment. Eosinophil percentage in induced sputum is a useful marker of airway eosinophilia. Inclusion of sputum cytology as a biomarker of airway eosinophilia in clinical asthma research will facilitate research on treatment responses and on mechanisms of disease, including mechanisms of disease in noneosinophilic asthma.

Analysis of the cell differential of induced sputum is a useful noninvasive method for evaluating airway inflammation in asthma.^69,71,72 In particular, the analysis of sputum eosinophils has proven valuable and has facilitated studies large enough to allow examination of the relationships between airway eosinophilia and measures of lung function. These studies have shown that sputum eosinophil percentage is related to measures of airflow obstruction and to measures of bronchial hyperresponsiveness.^73–75 Although sputum eosinophil numbers in patients with asthma are along a continuum, a large proportion of people with asthma consistently shows low numbers of eosinophils that are similar to the values found in people who do not have asthma.^73,75 These findings support data from bronchoscopy studies, which also identified patients with eosinophilic or noneosinophilic phenotypes of asthma.^76,77 The presence or absence of sputum eosinophilia can be determined using a 2% cutoff based on published reference values for eosinophils in induced sputum from healthy subjects; that is, subjects with 2% or greater sputum eosinophils have sputum eosinophilia, and subjects with less than 2% sputum eosinophils do not.^78,79 Compared with the noneosinophilic asthma phenotype, the eosinophilic asthma phenotype has more pronounced subepithelial fibrosis and is more responsive to inhaled corticosteroids.⁷⁷

Furthermore, recent studies have provided evidence that T_H2 cytokines orchestrate the eosinophilic asthma phenotype.⁸⁰ With the increasing emphasis on accurate phenotyping of asthma to optimize and personalize treatment programs,⁸¹ it is reasonable to propose that sputum eosinophils be quantified in characterizing study participants in asthma research studies. This information will allow a better understanding of how the eosinophil and non-eosinophil phenotypes of asthma influence responses to treatment interventions and also will facilitate mechanistic studies of these distinct phenotypes.

Range of values

Two published studies have provided reference values for sputum eosinophil percentages. In 1 study of 118 healthy nonsmoking subjects, the mean eosinophil percentage was 0.4 with an SD of 0.9, so that the upper range of normal calculated as the mean +2 SD was 2.2%.⁷⁸ In another study of 114 healthy subjects, the mean eosinophil percentage was 0.6 with an SD of 0.8, and a mean + 2 SD that was also 2.2%.⁷⁹ These data provide the rationale for a cutoff of 2% eosinophils to classify people with asthma as having sputum eosinophilia or not. Using sputum eosinophil analysis in this way has identified subgroups with asthma, with and without sputum eosinophilia.^77,82 The cell counts in sputum are presented as the nonsquamous cell percentage; squamous cells are counted independently to determine sample quality. A squamous cell percentage of greater than 80% is taken to indicate a sputum sample of inadequate quality.⁶⁹

Repeatability

The concordance correlation coefficient for the 1-week repeatability of sputum eosinophils has been reported by the Asthma Clinical Research Network to be 0.82 (95% CI, 0.72–0.88).⁸³

Responsiveness

Sputum cell counts can change within hours after an intervention, as has been shown in studies of airway allergen challenge or exposure to ozone.^84–86 The effects of currently available asthma treatments on sputum eosinophils vary. Treatment with corticosteroids consistently decreases sputum eosinophils.^87,88 Although leukotriene receptor antagonists have been shown to reduce sputum eosinophils,⁸⁹ these effects are not consistent⁸⁸ and are smaller compared with those of corticosteroids^90,91 or omalizumab.⁹² Long- or short-acting β-adrenergic agonists are not thought to alter sputum eosinophilia.

Validity

Sputum cell counts are a well-validated method for assessing cellular inflammation of the airways. Validation studies have included comparison with cell counts obtained during bronchoscopy⁹³ and studies done before and after interventions with corticosteroids,⁸⁷ aeroallergens,^84,85 and ozone,⁸⁶ as well as repeated measures and safety studies.^83,94

Associations

Inverse relationships have been observed between percentage of sputum eosinophils and FEV₁, as well as between percentage of sputum eosinophils and PC₂₀ to methacholine,⁷³ but sputum eosinophils alone do not account for all of the variability in these lung function measures among individuals who have asthma; other factors also appear to have strong influences. The relationship between blood and sputum eosinophils is likely complex and has not yet been well described.

Practicality and risk

Although researchers have proposed the use of sputum induction as a research tool in asthma for nearly 20 years, the procedure has not gained wide use in clinical practice because of logistic and practical difficulties. The test is sufficiently complex that it is only used as a clinical test for patient care in a few select centers. In contrast, the use of sputum induction and analysis of induced sputum is much more widespread in the research setting. Experience in research settings has taught us that the test is best used in centers that deploy it frequently and that its use in multicenter studies requires specific training and quality assurance programs.

The hypertonic saline used in sputum induction can cause bronchoconstriction, but pretreatment with albuterol prevents bronchoconstriction in most patients. To guard against excessive bronchoconstriction in some subgroups of patients, it is necessary to monitor lung function (usually by peak flow) during the induction procedure. When such monitoring is done, the procedure has been shown to have acceptable risk.⁹⁵

Demographic considerations

Sputum induction is feasible in children,⁹⁶ but because the test requires active participation by the subject, children under the age of 6 will have more difficulty completing it successfully.

Priority for NIH-initiated clinical research

Sputum eosinophil measurement is recommended as a supplemental outcome for the characterization of study populations, for prospective clinical trials, and for observational studies.

The strengths of this method include the fact that sputum cytology provides a direct measure of eosinophilic inflammation in the large airways. With increasing recognition of specific molecular phenotypes of asthma associated with specific cellular profiles, this advantage is becoming even more relevant. Coupled with emerging data that responsiveness to treatment such as corticosteroids or cytokine inhibitors depends on patterns of cellular inflammation in the airway^82,97,98 and that cellular inflammation is associated with specific patterns of airway remodeling,^76,77,80 it becomes clear that mechanism- or treatment-oriented studies in asthma are best done with full knowledge of the airway cytology phenotype.

Weaknesses include participant tolerance, the salty taste of the hypertonic saline, the small risk of bronchoconstriction, the difficulty in obtaining adequate samples in some study participants (particularly children), and the need to participate actively in sputum expectoration. These make the test unappealing to some participants. In addition, whereas the procedure is not very complex, it is probably too demanding for research in some settings, such as doctors’ offices. The combined expense of the induction and processing can add significantly to study costs. Finally, technician training is essential, because the test is not automated.

CBC/blood eosinophils

Summary

Analysis of blood eosinophils by automated CBC provides useful information to characterize study populations for prospective clinical trials and observational studies in asthma.
Blood eosinophils can be used as a biomarker to monitor systemic biological effects of pharmacologic and immunologic interventions in patients with asthma.

Definition and methodology for measurement

Venous blood is drawn and put into a tube containing EDTA. The CBC is determined using an automated analyzer, such as the Medonic M-Series, Beckman Coulter LH series, Roche Sysmex XE-2100, Siemens ADVIA 120 and 2120, Abbott CELL-DYN series, and Mindray BC series. The total number of white blood cells is multiplied by the percentage of eosinophils to provide the absolute eosinophil count (eosinophils × 10⁹/L). The percentage of eosinophils should not be reported unless specific reasons exist for knowing the proportions of eosinophils compared with other cells. Automated counting systems are accurate, but they can produce errors in samples with high blood eosinophil counts. Manual counting is not recommended because of inaccuracy concerns.⁹⁹

Medical and scientific value

The association between eosinophilia and asthma was observed shortly after eosinophils were first described.¹⁰⁰ In patients with asthma, blood eosinophil counts are often, but not always, increased.^101,102 In both children and adults, a direct correlation was observed between blood eosinophil counts and symptom scores,^103,104 and an inverse correlation was found with FEV₁.¹⁰⁴ Furthermore, eosinophil counts in adults correlated with the magnitude of bronchial hyperreactivity and diurnal peak expiratory flow variation.¹⁰⁴ Thus peripheral eosinophil counts may reflect asthma activity in both children and adults.

Historically, eosinophils have been considered effector cells involved in bronchial asthma and allergic diseases.¹⁰⁵ Activated eosinophils release toxic granule proteins and proinflammatory mediators that may cause tissue damage and dysfunction.¹⁰⁶ Eosinophils also may be involved in tissue remodeling and immunoregulation.¹⁰⁷ However, the roles of eosinophils in human asthma are still poorly understood.^108,109

Range of values

In adults, blood eosinophil counts range from 0.015 to 0.65 × 10⁹/L (95% confidence limits).¹¹⁰ In children 4 to 8 years of age, blood eosinophil counts average 0.206 ± 0.027 × 10⁹/L.¹¹¹ In children over 12 years of age, the counts are lower: for males, eosinophil counts average 0.180 ± 0.016 × 10⁹/L, and for females, they average 0.145 ± 0.012 × 10⁹/L.¹¹¹

Repeatability

Blood eosinophil counts vary diurnally in healthy individuals by more than 40%.¹¹² The counts are inversely related to blood cortisol levels; that is, they are lowest in the morning and highest at night. Therefore blood samples for eosinophil counts should always be collected at the same time of day. Exercise and smoking also increase the blood eosinophil count.^113,114

Responsiveness

The blood eosinophil count reflects various immunological and inflammatory parameters of asthma, such as blood and tissue levels of cytokines and chemokines. IL-4 and IL-13 play a central role in promoting eosinophil trafficking, whereas IL-5 is the major cytokine promoting eosinophil differentiation, proliferation, and activation.¹¹⁵ Thus blood eosinophil counts change in response to treatments that affect these parameters. For example, in patients with asthma, blood eosinophil counts start to decrease within 24 hours after intravenous administration of anti–IL-5 antibody, followed by much greater decreases several days later.^61,116,117 Treatment of asthmatic patients with anti-IgE antibody, leukotriene antagonists, or 5-lipoxygenese (5-LO) inhibitors reduces blood eosinophil counts.¹¹⁸ Conversely, blood eosinophils can persist in patients with corticosteroid-resistant asthma, perhaps identifying poor corticosteroid responsiveness.¹¹⁹

Validity

CBC with automated analyzers is a well-validated method for assessing the number of leukocytes in the blood.

Associations

An inverse correlation exists between the level of pulmonary function and the number of blood eosinophils.¹²⁰ Feno and peripheral blood eosinophils are elevated in patients with severe asthma with persistent airflow obstruction. However, no differences have been observed in the numbers of eosinophils between patients with asthma with elevated levels of IgE antibodies and those without elevated levels of IgE antibodies.¹²¹ The relation between blood and sputum eosinophils has not been reported.

Practicality and risk

CBC is a routine, standardized clinical test in medical institutions and clinical practices and is readily available. The variation in results between sites is none to minimal. The cost for the test is $10 to $13. Blood drawing and the blood volume necessary for CBC are considered to be of minimal risk.

Demographic considerations

CBC is feasible in both children and adults. Blood eosinophil counts in 4 ethnic groups (Asian-Indian, black, white, and non-Indian Asian) showed no significant differences.¹²²

Priority for NIH-initiated clinical research

Blood eosinophil counts are considered supplemental asthma biomarker measures for characterizing patients and outcome measures in clinical trials and observational studies, depending on the study question and design. This parameter is readily available in medical institutions, and the risk to patients with asthma is minimal. Analysis of eosinophil counts in peripheral blood provides a useful tool for characterizing/phenotyping study populations for prospective clinical trials and observational studies in asthma and for assessing the efficacy of certain pharmacologic and immunologic agents. Although the test has several weaknesses, as discussed below, the relative merit of blood eosinophil counts as an asthma outcome is high, considering the information gained, responsiveness, practical issues, and risk.

Blood eosinophil counts might provide additional unique information for asthma phenotyping, as compared to other asthma biomarkers. For example, in adults, correlations between the blood eosinophil count and the magnitude of airway hyperreactivity are noted irrespective of the presence of specific IgE antibodies.¹⁰⁴ Furthermore, increased blood eosinophil counts were observed in patients with asthma and with extensive sinus involvement by computed tomography (CT) scans.¹²³

Eosinophils are primarily tissue-dwelling leukocytes. Thus blood eosinophil counts do not necessarily indicate the extent of eosinophil involvement in affected tissues. The half-life of eosinophils in blood is short (ie, 18.0 ± 2.1 hours).¹²⁴ Thus the eosinophil count fluctuates considerably and is influenced by various factors, including exposure to allergens, treatment with inhaled and/or oral corticosteroids, and exposure to infectious agents or stress. After inhalation challenge with an allergen, patients who develop a late-phase reaction show an early decrease in blood eosinophils, followed by an increase.¹²⁵ Corticosteroids inhibit the development of eosinophils, although the acute fall in blood eosinophil counts in vivo caused by corticosteroids mainly results from the redistribution of the cells in the blood.¹²⁶ The rapid decrease in blood eosinophil counts following infection or stress involves both an increased uptake of eosinophils into tissues and a decreased output of eosinophils from bone marrow.¹²⁷

Urinary leukotriene E₄

Summary

Urinary leukotriene E₄ (LTE₄) is a validated marker of cysteinyl leukotriene activity and should be considered for incorporation in clinical trials of molecules that may directly or indirectly affect this pathway.

Definition and methodology for measurement

Cysteinyl leukotrienes are eicosanoids produced by a variety of cells associated with allergic inflammation, including eosinophils, mast cells, and basophils. The end metabolite of cysteinyl leukotrienes, LTE₄, can be generally measured in random urine samples. No clear benefit has been shown to measuring 24-hour samples. Mass spectroscopy is recommended as the method of measurement.^128,129

Medical and scientific value

Urinary LTE₄ is an indirect marker of lung cysteinyl leukotriene activity. Urinary LTE₄ increases with asthma exacerbations, aspirin and allergen challenges, and perhaps at night with nocturnal asthma.^130–133 Drugs that block cysteinyl leukotriene synthesis significantly decrease urinary LTE₄ levels,^134,135 whereas corticosteroids do not.^131,136 Levels have been shown to be greater in individuals with severe asthma whose disease onset occurs after 12 years of age, as compared to those with early onset, perhaps due to the more eosinophilic nature of some phenotypes of adult onset asthma, including aspirin-exacerbated respiratory disease.¹³⁷

Range of values

Values can vary dramatically, depending on methodology. However, both 2-step purification methods (using immunopreciptation and enzyme linked immunoassays) and mass spectroscopy have generally established normal levels to be less than 50 pg/μg (picograms per micrograms) creatinine.^128,129 In aspirin-exacerbated respiratory disease and other eosinophilic forms of severe asthma, levels can be much higher and may be measured in nanograms per microgram.^138,139

Repeatability

If a study participant’s asthma is stable, levels tend to be stable as well.¹⁴⁰ Levels increase following exposure to allergen, aspirin, and other nonsteroidal anti-inflammatory drugs in sensitive individuals, and in exacerbations. The levels decrease in the presence of 5-LO inhibitors.

Responsiveness

Responsiveness to treatment depends on the urinary LTE₄ starting level, with low levels less likely to show measureable responses to 5-LO inhibitors. However, in general, 5-LO inhibitors decrease urinary LTE₄ levels by 40% to 75%.^134,135 Levels increase in asthma exacerbations, as well as with aspirin and allergen challenge.^130,133 Corticosteroids have minimal effects on the levels.^131,136

Validity

Urinary LTE₄ is modestly associated with lung function measurements over time, as well as with a fall in FEV₁ during aspirin challenge and the degree of airway obstruction during an asthma exacerbation.^130,133,141 While many people who have asthma have increased LTE₄ during asthma exacerbations, the sensitivity and specificity of this measure across all patients with asthma is limited. Although 1 study in children suggested that urinary LTE₄ levels predicted response to leukotriene receptor antagonists, previous studies in adults did not demonstrate such a predictive value.¹⁴²

Associations

Increased basal levels of urinary LTE₄ have been associated with aspirin-sensitive asthma, adult-onset asthma, lung function, and blood eosinophils.^{137,141,143,144} However, the associations are not very strong, so LTE₄ cannot be considered a surrogate for these other measures.

Practicality and risk

Whereas collection of urine is simple and of no risk to study participants, the measurement of LTE₄ is not simple. Special equipment and training are required to conduct the recommended measurement approach of mass spectroscopy.¹²⁸

Demographic considerations

Higher urinary LTE₄ levels are seen in adult-onset severe asthma and aspirin-sensitive asthma.^137,143

Priority for NIH-initiated clinical research

Urinary LTE₄ is recommended as a supplemental outcome for the characterization of study populations, for prospective clinical trials, and for observational studies. The test should be strongly considered for inclusion in any study that attempts to manipulate the eicosanoid pathway. LTE₄ measurement also should be considered for studies that characterize asthma phenotypes, such as aspirin-sensitive, adult-onset, and eosinophilic asthma.

Future directions or research questions

Urinary LTE₄ was collected in most of the early leukotriene-modifying drug trials. However, the relationship of urinary leukotrienes to particular phenotypes, beyond aspirin sensitivity, has never been addressed. It is conceivable that the increased emphasis on asthma phenotyping may increase the importance of urinary LTE₄ measurement. Interventions that affect other aspects of eicosanoid biology (eg, COX) also should consider including measurement of urinary LTE₄ to determine whether this pathway is directly or indirectly affected.

EMERGING BIOMARKERS

Cortisol

Cortisol measures can be used in the following ways:

Cortisol suppression measures are used primarily as a biomarker to assess inhaled or systemic corticosteroids with respect to the level of systemic exposure and their effect on the hypothalamic-pituitary-adrenal axis.
Measurement of cortisol levels (particularly 12-hour overnight or 24-hour plasma cortisol) should be considered for the characterization and definition of the therapeutic index of new corticosteroids (asthma effect-to-systemic activity ratio).^57,145,146
Salivary cortisol measures can be used in studies to evaluate neuroendocrine effects of stress.^147–149

The preferred method for cortisol measurement is high-performance liquid chromatography.⁵⁷ Immunoassays have a high potential for interference from exogenous corticosteroids. Twenty-four-hour and overnight urinary free cortisol also have been used and are accepted by the FDA as a means of assessing systemic corticosteroid activity. Measurements of salivary cortisol have been used in studies of stress response but have not been standardized and applied to studies comparing systemic effects of oral and inhaled corticosteroids. In addition, some studies directly measure the corticosteroid of interest from plasma or serum to assess bioavailability.¹⁵⁰

When studying the effect of an inhaled or systemic corticosteroid on the hypothalamic-pituitary-adrenal axis, the degree of cortisol suppression is related to the dose and type of corticosteroid administered, as well as the delivery device for an inhaled corticosteroid.^57,145,150 While cortisol suppression by plasma or urinary free cortisol measurement is a sensitive measure of exogenous corticosteroid administration, the relationship to clinically significant suppression has not been established.¹⁴⁸ To date, cortisol suppression has not been shown to correlate well with clinically relevant measures of corticosteroid adverse effects. Salivary samples also may be used to assess cortisol suppression. Although saliva collection is convenient, especially for pediatric studies, precautions are needed when using this technique for clinical studies due to lack of standardization and lack of data on use for comparative studies.¹⁴⁷

Measurements of cortisol are considered emerging for asthma clinical trials involving corticosteroids. The strength of plasma or serum cortisol measurement is its sensitivity in identifying the systemic effect of corticosteroid therapy. However, studies are still needed to determine whether the degree of cortisol suppression is associated with long-term risk for clinically relevant adverse effects, such as reduced growth, osteoporosis, or cataracts.¹⁵¹

Some directions for future research related to the application of cortisol measurement include the development of convenient and reliable methods to assess cortisol suppression in young children because of the limitations in the volume of blood that can be collected given the frequency of sampling required for currently standardized measures. Therefore defining the relationship between blood and salivary cortisol would be useful, especially in children. Identifying the relationship of cortisol suppression to clinically relevant indicators of adverse corticosteroid effects is also important.

High-resolution CT scanning

High-resolution computed tomography (HRCT) can be used to measure airway lumen (diameter, area), airway wall (thickness, area), parenchymal density, and lung volume. HRCT images are used to measure specific airway narrowing, wall thickening, air trapping, and ventilation inhomogeneity in health and disease. Imaging allows assessment of the structure of the airways and parenchyma not obtainable by any other in vivo methods. HRCT airway lumen and wall measurements have been correlated with lung function and severity of asthma. In addition, increased parenchymal lucency has been associated with severe exacerbations of asthma, FEV₁, atopy, and neutrophilic inflammation.¹⁵³ HRCT is an emerging outcome measure for NIH-initiated clinical research.

HRCT has no single preferred method or scanner. Multiple scanners, manufacturers, and scanning parameters have been used. HRCT is easy to perform, and the scanners and personnel are readily available. The 1 risk is exposure to ionizing radiation.¹⁵² Validity and reproducibility, particularly for airway measurements, require that, within each study participant, the lungs are scanned at a standard volume.

Pixel counting is the common method of measurement for HRCT. In addition, pixel intensity is used for parenchymal measurements of air content. The various methods have been validated via phantoms of parenchyma and airways of various sizes and density. HRCT measurements have been shown to be repeatable in animal models, although studies in humans with and without lung disease are lacking. The magnitude of change is usually measured as a percentage change in airway luminal size or airway wall thickness or the percentage of parenchymal density measures below a certain threshold (eg, −856 Hounsfield units for air trapping). However, a clinically relevant magnitude of change has not been determined. Furthermore, no normal ranges have been established for any of the HRCT imaging measurements. The effects of gender, age, or race/ethnicity on HRCT measurements have not been determined.

More validation and reproducibility data are required before HRCT can move from the category of an emerging outcome.

Sputum neutrophils and analytes

Additional emerging outcomes include sputum neutrophil evaluation and various analytes measured in the sputum sol phase. Neutrophilic asthma (generally defined through sputum evaluation) has been proposed as a phenotype associated with more severe disease, lower lung function, corticosteroid use (and poor response to this treatment), asthma exacerbations, and smoking.^154–157 However, sputum neutrophilia is not specific for asthma, being observed in numerous other lung diseases. There is variability in thresholds for neutrophilia, depending on sputum processing and centers, ranging from 40% to 65%.^158,159 A recent study suggested considerable temporal variability without clinical association, while another suggested that when controlling for corticosteroid use and smoking, the phenotype did not exist.^159,160 Similarly, a variety of different analytes have been measured in sputum supernatants. However, to date, the sample sizes are small, and there are no reproducibility studies. For these reasons, sputum neutrophils and measurements of analytes are categorized as emerging biomarkers.

Exhaled breath condensate

Exhaled breath condensate (EBC) is a composite volatile and droplet lung collection that may allow for noninvasive assessment of biochemistry and inflammation, with the relative contributions to EBC from proximal versus distal airways still unclear. Because of the numerous assays available to perform on EBC, with often only a few investigators studying any 1 biomarker, there have been only modest efforts at validation studies on any 1 biomarker. EBC pH is the most technically validated of the assays and focuses on a biochemical disturbance common in inflammatory diseases in general.¹⁶¹ If airway neutralization therapies prove useful for subgroups of subjects with asthma symptoms, then EBC pH may become a particularly important biomarker. Likewise, careful use of EBC pH may allow identification of acute acid reflux events.¹⁶² Nitrogen oxides, hydrogen peroxide, glutathione, aldehydes, isoprostanes, and pH in EBC may provide more information regarding airway oxidative stresses than other approaches, although confidence in interpretation remains modest. Additionally, assessment of airways inflammation with EBC assays for cytokines, leukotrienes, prostaglandins, adenosine, and others has been reported.¹⁶³ Assays for such nonvolatile substances can be improved by controlling for the dilution of airway lining fluid during the collection process.¹⁶⁴ However, these assays remain insufficiently controlled and standardized, and therefore are emerging biomarkers of interest.

Biomarker discovery through genetics and genomic profiling

Biomarker discovery and validation using large clinical trial populations is now possible with the use of genetics and genomic profiling (eg, transcriptomics, proteomics, lipomics, and metabolomics). However, there are several issues to consider relative to the acquisition and storage of samples, as discussed below.

BIOSPECIMEN ACQUISITION AND STORAGE FOR DETECTION OF BIOMARKERS

Most clinical trials archive biospecimens for genetics and genomics (eg, transcriptomics, proteomics, lipomics, and metabolomics); however, the stability of the biospecimens over extended periods remains unclear. Generally, storage at −80°C immediately upon acquisition confers the greatest stability. Evidence suggests that samples obtained from study participants with asthma are more susceptible to degradation than those obtained from control participants.^165,166 Peripheral blood, the most accessible source for various transcriptomic and proteomic studies, is separated to obtain plasma or serum or processed to obtain the buffy coat for DNA analysis. Limitations of peripheral blood analysis relate to the systemic nature of blood and the extent to which it reflects the lung and/or airway compartment. An excellent resource for biospecimen preparation is the standard operating procedures of the National Cancer Institute’s Early Detection Research Network (http://edrn.nci.nih.gov/resources). Blood samples processed with these procedures are stable at −70°C for more than 3 years, although differences in stability between disease and control samples remain to be determined. Clinical blood collection transcriptome analysis remains challenging, with degradation of transcripts and even induction of gene expression by sample handling.¹⁶⁷ The speed of acquisition and maintenance of aliquots at −80°C are likely to be the most important factors in determining stability over time,^165,168 and cloned (c) DNA is considered to be more stable than RNA.

Urine also serves as a convenient and stable platform for the performance of metabolomics, proteomics, and lipidomics. Samples should be spun quickly and snap frozen.^169,170 Unfortunately, although urine is easily acquired, urinary changes may not adequately reflect events occurring in the lung or the airways.

Sputum and EBC, important resources for biomarker discovery, may directly correlate with lung pathology. However, many technical challenges remain before sputum analysis and EBC can be routinely used and sufficiently validated in discovery studies.^{165,171–173}

Standard operating procedures for harvesting body fluids are essential to harmonize protocols. Studies are needed to compare approaches for storing blood, urine, sputum, and other airway fluids to determine the optimal approaches to maximize yield without loss of fidelity. Validation and reproducibility studies also are required before stability of biospecimen storage can be assured. The adequate storage and quality assurance of biospecimens should be a priority for collaborative, multicenter studies in asthma. Central storage facilities will likely decrease biospecimen variance and improve quality. Collective research efforts must focus on comparing the quality and stability of biospecimens from large central repositories, such as the National Heart, Lung, and Blood Institute biorepository (Biologic Specimen and Data Repository Information Coordinating Center, or BioLINCC; see https://biolincc.nhlbi.nih.gov/home), to ensure maximal use of these valuable resources. Ethical questions regarding the use of these biospecimens several years after collection need to be thoroughly discussed and consensus reached.

PRIORITY QUESTIONS FOR FUTURE RESEARCH

Can exhaled NO best be used as a unique marker of inflammation to predict and monitor response to asthma treatment?
Do differences in sputum processing significantly affect the utility of sputum eosinophils to guide anti-inflammatory therapy?
Can a simpler surrogate for sputum eosinophils be developed?
Will characterization of asthma by atopic status contribute to a better understanding and differentiation of asthma phenotypes?
What is the relationship between measures of cortisol suppression and meaningful systemic effects of corticosteroid therapy?
Can lung imaging be standardized and used to define airway remodeling in asthma?
Can future studies emphasize a design in which the patient population is selected on the basis of biomarker-based phenotypes?

TABLE III

Key points and recommendations

Acknowledgments

The Asthma Outcomes workshop was funded by contributions from the National Institute of Allergy and Infectious Diseases; the National Heart, Lung, and Blood Institute; the Eunice Kennedy Shriver National Institute of Child Health and Human Development; the National Institute of Environmental Health Sciences; the Agency for Healthcare Research and Quality; and the Merck Childhood Asthma Network, as well as by a grant from the Robert Wood Johnson Foundation. Contributions from the National Heart, Lung, and Blood Institute; the National Institute of Allergy and Infectious Diseases; the Eunice Kennedy Shriver National Institute of Child Health and Human Development; the National Institute of Environmental Health Sciences; and the US Environmental Protection Agency funded the publication of this article and all other articles in this supplement.

Abbreviations used


ATS	American Thoracic Society

CBC	Complete blood count

CT	Computed tomography

EBC	Exhaled breath condensate

ERS	European Respiratory Society

FDA	US Food and Drug Administration

Feno	Fractional exhaled nitric oxide

5-LO	5-Lipoxygenase

HRCT	High-resolution computed tomography

LTE₄	Leukotriene E₄

NIH	National Institutes of Health

NO	Nitric oxide

Footnotes

Disclosure of potential conflict of interest: S. J. Szefler is a consultant for GlaxoSmith-Kline, Genentech, Merck, Boehringer-Ingelheim, Novartis, and Schering-Plough; and has received research support from the NIH (NHLBI, NIAID, NIEHS) and the Environmental Protection Agency. J. V. Fahy is on the Cytokinetics Scientific advisory board; has received consulting fees from Amgen, Gilcad, Five Prime Therapeutics, Merck, Regeneron Pharmaceuticals, Portola Pharmaceuticals; and has received research support from the NIH (NHLBI and NIAID) and Genentech. J. F. Hunt is on the Pulse Health LLC advisory board; is founder of Respiratory Research, Inc; has received research support from the NIH-NIAID and Altrea; and is Chair of the AAAAI Asthma Diagnosis and Pharmacotherapeutics Committee and Cough Committee. A. H. Liu has received speaker honoraria from Merck; is on the Data Safety Monitoring Board for GlaxoSmithKline; and is a consultant for DBV. R. A. Panettieri, Jr, is a consultant for Johnson & Johnson, Forest Pharmaceuticals, Genentech, and Merck; and has received research support from AstraZeneca, Johnson & Johnson, Merck, and Roche. R. P. Schleimer is a consultant for GlaxoSmithKline and Intersect ENT. The rest of the authors declare that they have no relevant conflicts of interest.

References

1. American Thoracic Society. Recommendations for standardized procedures for the on-line and off-line measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide in adults and children–1999. This official statement of the American Thoracic Society was adopted by the ATS Board of Directors, July 1999. Am J Respir Crit Care Med. 1999;160(6):2104–17. Epub 1999/12/10. [PubMed]

2. American Thoracic Society; European Respiratory Society. ATS/ERS recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide, 2005. Am J Respir Crit Care Med. 2005;171(8):912–30. Epub 2005/04/09. [PubMed]

3. Dweik RA, Boggs PB, Erzurum SC, Irvin CG, Leigh MW, Lundberg JO, et al. An official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels (FENO) for clinical applications. Am J Respir Crit Care Med. 2011;184(5):602–15. Epub 2011/09/03. [PubMed]

4. Mattke S, Martorell F, Sharma P, Malveaux F, Lurie N. Quality of care for childhood asthma: estimating impact and implications. Pediatrics. 2009;123(Suppl 3):S199–204. Epub 2009/04/16. [PubMed]

5. Reddel HK, Taylor DR, Bateman ED, Boulet LP, Boushey HA, Busse WW, et al. An official American Thoracic Society/European Respiratory Society statement: asthma control and exacerbations: standardizing endpoints for clinical asthma trials and clinical practice. Am J Respir Crit Care Med. 2009;180(1):59–99. Epub 2009/06/19. [PubMed]

6. Holgate ST, Lemanske RF, Jr, O’Byrne PM, Kakumanu S, Busse WW. Asthma pathogenesis. In: Adkinson NF, Middleton E, editors. Middleton’s allergy: principles & practice. 7. Philadelphia (PA): Mosby Elsevier; 2009. pp. 893–919.

7. Huggins KG, Brostoff J. Local production of specific IgE antibodies in allergic-rhinitis patients with negative skin tests. Lancet. 1975;2(7926):148–50. Epub 1975/07/26. [PubMed]

8. Merrett TG, Houri M, Mayer AL, Merrett J. Measurement of specific IgE antibodies in nasal secretion–evidence for local production. Clin Allergy. 1976;6(1):69–73. Epub 1976/01/01. [PubMed]

9. Small P, Barrett D, Frenkiel S, Rochon L, Cohen C, Black M. Local specific IgE production in nasal polyps associated with negative skin tests and serum RAST. Ann Allergy. 1985;55(5):736–9. Epub 1985/11/01. [PubMed]

10. Shatkin JS, Delsupehe KG, Thisted RA, Corey JP. Mucosal allergy in the absence of systemic allergy in nasal polyposis and rhinitis: a meta-analysis. Otolaryngol Head Neck Surg. 1994;111(5):553–6. Epub 1994/11/01. [PubMed]

11. Yoshida T, Usui A, Kusumi T, Inafuku S, Sugiyama T, Koide N, et al. A quantitative analysis of cedar pollen-specific immunoglobulins in nasal lavage supported the local production of specific IgE, not of specific IgG. Microbiol Immunol. 2005;49(6):529–34. Epub 2005/06/21. [PubMed]

12. Burrows B, Martinez FD, Cline MG, Lebowitz MD. The relationship between parental and children’s serum IgE and asthma. Am J Respir Crit Care Med. 1995;152(5 Pt 1):1497–500. Epub 1995/11/01. [PubMed]

13. Sherrill DL, Lebowitz MD, Halonen M, Barbee RA, Burrows B. Longitudinal evaluation of the association between pulmonary function and total serum IgE. Am J Respir Crit Care Med. 1995;152(1):98–102. Epub 1995/07/01. [PubMed]

14. Burrows B, Martinez FD, Halonen M, Barbee RA, Cline MG. Association of asthma with serum IgE levels and skin-test reactivity to allergens. N Engl J Med. 1989;320(5):271–7. Epub 1989/02/02. [PubMed]

15. Hamilton R. Human immunoglobulins. In: O’Gorman MRG, Donnenberg AD, editors. Handbook of human immunology. 2. Boca Raton (FL): CRC Press; 2008. pp. 63–106.

16. Matsson P, Hamilton RG, Esch RE, Homburger HA, Kleine-Tebbe J, Mari A, et al. Analytical performance characteristics and clinical utility of immunological assays for human immunoglobulin E (IgE) antibody and defined allergen specificities. Wayne (PA): Clinical and Laboratory Standards Institute; 2009.

17. Sly PD, Boner AL, Bjorksten B, Bush A, Custovic A, Eigenmann PA, et al. Early identification of atopy in the prediction of persistent asthma in children. Lancet. 2008;372(9643):1100–6. Epub 2008/09/23. [PubMed]

18. Simpson A, Soderstrom L, Ahlstedt S, Murray CS, Woodcock A, Custovic A. IgE antibody quantification and the probability of wheeze in preschool children. J Allergy Clin Immunol. 2005;116(4):744–9. Epub 2005/10/08. [PubMed]

19. Wickman M, Ahlstedt S, Lilja G, van Hage Hamsten M. Quantification of IgE antibodies simplifies the classification of allergic diseases in 4-year-old children. A report from the prospective birth cohort study–BAMSE. Pediatr Allergy Immunol. 2003;14(6):441–7. Epub 2003/12/17. [PubMed]

20. Wickman M, Lilja G, Soderstrom L, van Hage-Hamsten M, Ahlstedt S. Quantitative analysis of IgEantibodies tofood and inhalant allergens in 4-year-old children reflects their likelihood of allergic disease. Allergy. 2005;60(5):650–7. Epub 2005/04/09. [PubMed]

21. Hamilton RG. Clinical laboratory assessment of immediate-type hypersensitivity. J Allergy Clin Immunol. 2010;125(2 Suppl 2):S284–96. Epub 2010/03/05. [PubMed]

22. Hamilton RG. Proficiency survey-based evaluation of clinical total and allergen-specific IgE assay performance. Arch Pathol Lab Med. 2010;134(7):975–82. Epub 2010/07/01. [PubMed]

23. Wood RA, Phipatanakul W, Hamilton RG, Eggleston PA. A comparison of skin prick tests, intradermal skin tests, and RASTs in the diagnosis of cat allergy. J Allergy Clin Immunol. 1999;103(5 Pt 1):773–9. Epub 1999/05/18. [PubMed]

24. Williams PB, Siegel C, Portnoy J. Efficacy of a single diagnostic test for sensitization to common inhalant allergens. Ann Allergy Asthma Immunol. 2001;86(2):196–202. Epub 2001/03/22. [PubMed]

25. Garcia-Marcos L, Sanchez-Solis M, Martinez-Torres AE, Lucas Moreno JM, Sastre VH. Phadiatop compared to skin-prick test as a tool for diagnosing atopy in epidemiological studies in schoolchildren. Pediatr Allergy Immunol. 2007;18(3):240–4. [PubMed]

26. Diaz-Vazquez C, Torregrosa-Bertet MJ, Carvajal-Uruena I, Cano-Garcinuno A, Fos-Escriva E, Garcia-Gallego A, et al. Accuracy of ImmunoCAP Rapid in the diagnosis of allergic sensitization in children between 1 and 14 years with recurrent wheezing: the IReNE study. Pediatr Allergy Immunol. 2009;20(6):601–9. Epub 2009/02/18. [PubMed]

27. Halvorsen R, Jenner A, Hagelin EM, Borres MP. Phadiatop infant in the diagnosis of atopy in children with allergy-like symptoms. Int J Pediatr. 2009:460737. Epub 2009/12/31. [PMC free article] [PubMed]

28. McCann WA, Ownby DR. The reproducibility of the allergy skin test scoring and interpretation by board-certified/board-eligible allergists. Ann Allergy Asthma Immunol. 2002;89(4):368–71. Epub 2002/10/24. [PubMed]

29. Hamilton RG. Accuracy of US Food and Drug Administration-cleared IgE antibody assays in the presence of anti-IgE (omalizumab) J Allergy Clin Immunol. 2006;117(4):759–66. Epub 2006/04/25. [PubMed]

30. Zieg G, Lack G, Harbeck RJ, Gelfand EW, Leung DY. In vivo effects of glucocorticoids on IgE production. J Allergy Clin Immunol. 1994;94(2 Pt 1):222–30. Epub 1994/08/01. [PubMed]

31. Jabara HH, Ahern DJ, Vercelli D, Geha RS. Hydrocortisone and IL-4 induce IgE isotype switching in human B cells. J Immunol. 1991;147(5):1557–60. Epub 1991/09/01. [PubMed]

32. Sears MR, Burrows B, Flannery EM, Herbison GP, Hewitt CJ, Holdaway MD. Relation between airway responsiveness and serum IgE in children with asthma and in apparently normal children. N Engl J Med. 1991;325(15):1067–71. [PubMed]

33. Barbee RA, Halonen M, Lebowitz M, Burrows B. Distribution of IgE in a community population sample: correlations with age, sex, and allergen skin test reactivity. J Allergy Clin Immunol. 1981;68(2):106–11. Epub 1981/08/01. [PubMed]

34. Kharitonov SA, Gonio F, Kelly C, Meah S, Barnes PJ. Reproducibility of exhaled nitric oxide measurements in healthy and asthmatic adults and children. Eur Respir J. 2003;21(3):433–8. Epub 2003/03/29. [PubMed]

35. Olivieri M, Talamini G, Corradi M, Perbellini L, Mutti A, Tantucci C, et al. Reference values for exhaled nitric oxide (reveno) study. Respir Res. 2006;7:94. Epub 2006/07/04. [PMC free article] [PubMed]

36. Olin AC, Bake B, Toren K. Fraction of exhaled nitric oxide at 50 mL/s: reference values for adult lifelong never-smokers. Chest. 2007;131(6):1852–6. Epub 2007/06/15. [PubMed]

37. Travers J, Marsh S, Aldington S, Williams M, Shirtcliffe P, Pritchard A, et al. Reference ranges for exhaled nitric oxide derived from a random community survey of adults. Am J Respir Crit Care Med. 2007;176(3):238–42. Epub 2007/05/05. [PubMed]

38. Dressel H, de la Motte D, Reichert J, Ochmann U, Petru R, Angerer P, et al. Exhaled nitric oxide: independent effects of atopy, smoking, respiratory tract infection, gender and height. Respir Med. 2008;102(7):962–9. Epub 2008/04/09. [PubMed]

39. Baraldi E, Azzolin NM, Cracco A, Zacchello F. Reference values of exhaled nitric oxide for healthy children 6–15 years old. Pediatr Pulmonol. 1999;27(1):54–8. Epub 1999/02/19. [PubMed]

40. Buchvald F, Baraldi E, Carraro S, Gaston B, De Jongste J, Pijnenburg MW, et al. Measurements of exhaled nitric oxide in healthy subjects age 4 to 17 years. J Allergy Clin Immunol. 2005;115(6):1130–6. Epub 2005/06/09. [PubMed]

41. Buchvald F, Hermansen MN, Nielsen KG, Bisgaard H. Exhaled nitric oxide predicts exercise-induced bronchoconstriction in asthmatic school children. Chest. 2005;128(4):1964–7. Epub 2005/10/21. [PubMed]

42. Kovesi T, Kulka R, Dales R. Exhaled nitric oxide concentration is affected by age, height, and race in healthy 9- to 12-year-old children. Chest. 2008;133(1):169–75. Epub 2007/10/11. [PubMed]

43. Wong GW, Liu EK, Leung TF, Yung E, Ko FW, Hui DS, et al. High levels and gender difference of exhaled nitric oxide in Chinese schoolchildren. Clin Exp Allergy. 2005;35(7):889–93. Epub 2005/07/13. [PubMed]

44. Malmberg LP, Petays T, Haahtela T, Laatikainen T, Jousilahti P, Vartiainen E, et al. Exhaled nitric oxide in healthy nonatopic school-age children: determinants and height-adjusted reference values. Pediatr Pulmonol. 2006;41(7):635–42. Epub 2006/05/17. [PubMed]

45. Olin AC, Rosengren A, Thelle DS, Lissner L, Bake B, Toren K. Height, age, and atopy are associated with fraction of exhaled nitric oxide in a large adult general population sample. Chest. 2006;130(5):1319–25. Epub 2006/11/14. [PubMed]

46. Ekroos H, Karjalainen J, Sarna S, Laitinen LA, Sovijarvi AR. Short-term variability of exhaled nitric oxide in young male patients with mild asthma and in healthy subjects. Respir Med. 2002;96(11):895–900. Epub 2002/11/07. [PubMed]

47. Ekroos H, Tuominen J, Sovijarvi AR. Exhaled nitric oxide and its long-term variation in healthy non-smoking subjects. Clin Physiol. 2000;20(6):434–9. Epub 2000/12/02. [PubMed]

48. Latzin P, Beck J, Griese M. Exhaled nitric oxide in healthy children: variability and a lack of correlation with atopy. Pediatr Allergy Immunol. 2002;13(1):37–46. Epub 2002/05/10. [PubMed]

49. Pijnenburg MW, Floor SE, Hop WC, De Jongste JC. Daily ambulatory exhaled nitric oxide measurements in asthma. Pediatr Allergy Immunol. 2006;17(3):189–93. Epub 2006/05/05. [PubMed]

50. Warke TJ, Fitch PS, Brown V, Taylor R, Lyons JD, Ennis M, et al. Exhaled nitric oxide correlates with airway eosinophils in childhood asthma. Thorax. 2002;57(5):383–7. Epub 2002/04/30. [PMC free article] [PubMed]

51. Payne DN, Adcock IM, Wilson NM, Oates T, Scallan M, Bush A. Relationship between exhaled nitric oxide and mucosal eosinophilic inflammation in children with difficult asthma, after treatment with oral prednisolone. Am J Respir Crit Care Med. 2001;164(8 Pt 1):1376–81. Epub 2001/11/13. [PubMed]

52. Mattes J, Storm van’s Gravesande K, Reining U, Alving K, Ihorst G, Henschen M, et al. NO in exhaled air is correlated with markers of eosinophilic airway inflammation in corticosteroid-dependent childhood asthma. Eur Respir J. 1999;13(6):1391–5. Epub 1999/08/13. [PubMed]

53. Jatakanon A, Lim S, Kharitonov SA, Chung KF, Barnes PJ. Correlation between exhaled nitric oxide, sputum eosinophils, and methacholine responsiveness in patients with mild asthma. Thorax. 1998;53(2):91–5. Epub 1998/06/13. [PMC free article] [PubMed]

54. Jones SL, Kittelson J, Cowan JO, Flannery EM, Hancox RJ, McLachlan CR, et al. The predictive value of exhaled nitric oxide measurements in assessing changes in asthma control. Am J Respir Crit Care Med. 2001;164(5):738–43. Epub 2001/09/11. [PubMed]

55. Berry MA, Shaw DE, Green RH, Brightling CE, Wardlaw AJ, Pavord ID. The use of exhaled nitric oxide concentration to identify eosinophilic airway inflammation: an observational study in adults with asthma. Clin Exp Allergy. 2005;35(9):1175–9. Epub 2005/09/17. [PubMed]

56. Pijnenburg MW, Bakker EM, Lever S, Hop WC, De Jongste JC. High fractional concentration of nitric oxide in exhaled air despite steroid treatment in asthmatic children. Clin Exp Allergy. 2005;35(7):920–5. Epub 2005/07/13. [PubMed]

57. Szefler SJ, Martin RJ, King TS, Boushey HA, Cherniack RM, Chinchilli VM, et al. Significant variability in response to inhaled corticosteroids for persistent asthma. J Allergy Clin Immunol. 2002;109(3):410–8. Epub 2002/03/19. [PubMed]

58. Zacharasiewicz A, Wilson N, Lex C, Erin EM, Li AM, Hansel T, et al. Clinical use of noninvasive measurements of airway inflammation in steroid reduction in children. Am J Respir Crit Care Med. 2005;171(10):1077–82. [PubMed]

59. Berry M, Hargadon B, Morgan A, Shelley M, Richter J, Shaw D, et al. Alveolar nitric oxide in adults with asthma: evidence of distal lung inflammation in refractory asthma. Eur Respir J. 2005;25(6):986–91. Epub 2005/06/03. [PubMed]

60. Brightling CE, Symon FA, Birring SS, Bradding P, Wardlaw AJ, Pavord ID. Comparison of airway immunopathology of eosinophilic bronchitis and asthma. Thorax. 2003;58(6):528–32. Epub 2003/05/31. [PMC free article] [PubMed]

61. Haldar P, Brightling CE, Hargadon B, Gupta S, Monteiro W, Sousa A, et al. Mepolizumab and exacerbations of refractory eosinophilic asthma. N Engl J Med. 2009;360(10):973–84. Epub 2009/03/07. [PubMed]

62. Gratziou C, Lignos M, Dassiou M, Roussos C. Influence of atopy on exhaled nitric oxide in patients with stable asthma and rhinitis. Eur Respir J. 1999;14(4):897–901. Epub 1999/11/26. [PubMed]

63. Ho LP, Wood FT, Robson A, Innes JA, Greening AP. Atopy influences exhaled nitric oxide levels in adult asthmatics. Chest. 2000;118(5):1327–31. Epub 2000/11/18. [PubMed]

64. van den Toorn LM, Overbeek SE, De Jongste JC, Leman K, Hoogsteden HC, Prins JB. Airway inflammation is present during clinical remission of atopic asthma. Am J Respir Crit Care Med. 2001;164(11):2107–13. [PubMed]

65. van den Toorn LM, Prins JB, Overbeek SE, Hoogsteden HC, de Jongste JC. Adolescents in clinical remission of atopic asthma have elevated exhaled nitric oxide levels and bronchial hyperresponsiveness. Am J Respir Crit Care Med. 2000;162(3):953–7. [PubMed]

66. Taylor DR, Mandhane P, Greene JM, Hancox RJ, Filsell S, McLachlan CR, et al. Factors affecting exhaled nitric oxide measurements: the effect of sex. Respir Res. 2007;8:82. Epub 2007/11/17. [PMC free article] [PubMed]

67. Levesque MC, Hauswirth DW, Mervin-Blake S, Fernandez CA, Patch KB, Alexander KM, et al. Determinants of exhaled nitric oxide levels in healthy, nonsmoking African American adults. J Allergy Clin Immunol. 2008;121(2):396–402. e3. Epub 2007/11/27. [PubMed]

68. Komakula S, Khatri S, Mermis J, Savill S, Haque S, Rojas M, et al. Body mass index is associated with reduced exhaled nitric oxide and higher exhaled 8-isoprostanes in asthmatics. Respir Res. 2007;8:32. Epub 2007/04/18. [PMC free article] [PubMed]

69. Fahy JV, Liu J, Wong H, Boushey HA. Cellular and biochemical analysis of induced sputum from asthmatic and from healthy subjects. Am Rev Respir Dis. 1993;147(5):1126–31. Epub 1993/05/01. [PubMed]

70. Gershman NH, Wong HH, Liu JT, Mahlmeister MJ, Fahy JV. Comparison of two methods of collecting induced sputum in asthmatic subjects. Eur Respir J. 1996;9(12):2448–53. Epub 1996/12/01. [PubMed]

71. Pin I, Gibson PG, Kolendowicz R, Girgisgabardo A, Denburg JA, Hargreave FE, et al. Use of induced sputum cell counts to investigate airway inflammation in asthma. Thorax. 1992;47(1):25–9. [PMC free article] [PubMed]

72. Kips JC, Fahy JV, Hargreave FE, Ind PW, in’t Veen JC. Methods for sputum induction and analysis of induced sputum: a method for assessing airway inflammation in asthma. Eur Respir J Suppl. 1998;26:9S–12S. Epub 1998/05/20. [PubMed]

73. Woodruff PG, Khashayar R, Lazarus SC, Janson S, Avila P, Boushey HA, et al. Relationship between airway inflammation, hyperresponsiveness, and obstruction in asthma. J Allergy Clin Immunol. 2001;108(5):753–8. Epub 2001/11/03. [PubMed]

74. Polosa R, Renaud L, Cacciola R, Prosperini G, Crimi N, Djukanovic R. Sputum eosinophilia is more closely associated with airway responsiveness to bradykinin than methacholine in asthma. Eur Respir J. 1998;12(3):551–6. Epub 1998/10/08. [PubMed]

75. Louis R, Lau LC, Bron AO, Roldaan AC, Radermecker M, Djukanovic R. The relationship between airways inflammation and asthma severity. Am J Respir Crit Care Med. 2000;161(1):9–16. Epub 2000/01/05. [PubMed]

76. Wenzel SE, Schwartz LB, Langmack EL, Halliday JL, Trudeau JB, Gibbs RL, et al. Evidence that severe asthma can be divided pathologically into two inflammatory subtypes with distinct physiologic and clinical characteristics. Am J Respir Crit Care Med. 1999;160(3):1001–8. [PubMed]

77. Berry M, Morgan A, Shaw DE, Parker D, Green R, Brightling C, et al. Pathological features and inhaled corticosteroid response of eosinophilic and non-eosinophilic asthma. Thorax. 2007;62(12):1043–9. Epub 2007/03/16. [PMC free article] [PubMed]

78. Belda J, Leigh R, Parameswaran K, O’Byrne PM, Sears MR, Hargreave FE. Induced sputum cell counts in healthy adults. Am J Respir Crit Care Med. 2000;161(2 Pt 1):475–8. Epub 2000/02/15. [PubMed]

79. Spanevello A, Confalonieri M, Sulotto F, Romano F, Balzano G, Migliori GB, et al. Induced sputum cellularity. Reference values and distribution in normal volunteers. Am J Respir Crit Care Med. 2000;162(3 Pt 1):1172–4. Epub 2000/09/16. [PubMed]

80. Woodruff PG, Modrek B, Choy DF, Jia G, Abbas AR, Ellwanger A, et al. T-helper type 2-driven inflammation defines major subphenotypes of asthma. Am J Respir Crit Care Med. 2009;180(5):388–95. Epub 2009/06/02. [PMC free article] [PubMed]

81. Haldar P, Pavord ID, Shaw DE, Berry MA, Thomas M, Brightling CE, et al. Cluster analysis and clinical asthma phenotypes. Am J Respir Crit Care Med. 2008;178(3):218–24. Epub 2008/05/16. [PubMed]

82. Pavord ID, Brightling CE, Woltmann G, Wardlaw AJ. Non-eosinophilic corticosteroid unresponsive asthma. Lancet. 1999;353(9171):2213–4. [PubMed]

83. Fahy JV, Boushey HA, Lazarus SC, Mauger EA, Cherniack RM, Chinchilli VM, et al. Safety and reproducibility of sputum induction in asthmatic subjects in a multicenter study. Am J Respir Crit Care Med. 2001;163(6):1470–5. Epub 2001/05/24. [PubMed]

84. Pin I, Freitag AP, O’Byrne PM, Girgis-Gabardo A, Watson RM, Dolovich J, et al. Changes in the cellular profile of induced sputum after allergen-induced asthmatic responses. Am Rev Respir Dis. 1992;145(6):1265–9. Epub 1992/06/01. [PubMed]

85. Fahy JV, Liu J, Wong H, Boushey HA. Analysis of cellular and biochemical constituents of induced sputum after allergen challenge: a method for studying allergic airway inflammation. J Allergy Clin Immunol. 1994;93(6):1031–9. Epub 1994/06/01. [PubMed]

86. Fahy JV, Wong HH, Liu JT, Boushey HA. Analysis of induced sputum after air and ozone exposures in healthy subjects. Environ Res. 1995;70(2):77–83. Epub 1995/08/01. [PubMed]

87. Claman DM, Boushey HA, Liu J, Wong H, Fahy JV. Analysis of induced sputum to examine the effects of prednisone on airway inflammation in asthmatic subjects. J Allergy Clin Immunol. 1994;94(5):861–9. Epub 1994/11/01. [PubMed]

88. Boushey HA, Sorkness CA, King TS, Sullivan SD, Fahy JV, Lazarus SC, et al. Daily versus as-needed corticosteroids for mild persistent asthma. N Engl J Med. 2005;352(15):1519–28. Epub 2005/04/15. [PubMed]

89. Pizzichini E, Leff JA, Reiss TF, Hendeles L, Boulet LP, Wei LX, et al. Montelukast reduces airway eosinophilic inflammation in asthma: a randomized, controlled trial. Eur Respir J. 1999;14(1):12–8. Epub 1999/09/18. [PubMed]

90. Kanniess F, Richter K, Bohme S, Jorres RA, Magnussen H. Montelukast versus fluticasone: effects on lung function, airway responsiveness and inflammation in moderate asthma. Eur Respir J. 2002;20(4):853–8. Epub 2002/11/05. [PubMed]

91. Jayaram L, Duong M, Pizzichini MM, Pizzichini E, Kamada D, Efthimiadis A, et al. Failure of montelukast to reduce sputum eosinophilia in high-dose corticosteroid-dependent asthma. Eur Respir J. 2005;25(1):41–6. Epub 2005/01/11. [PubMed]

92. Djukanovic R, Wilson SJ, Kraft M, Jarjour NN, Steel M, Chung KF, et al. Effects of treatment with anti-immunoglobulin E antibody omalizumab on airway inflammation in allergic asthma. Am J Respir Crit Care Med. 2004;170(6):583–93. Epub 2004/06/03. [PubMed]

93. Fahy JV, Wong H, Liu J, Boushey HA. Comparison of samples collected by sputum induction and bronchoscopy from asthmatic and healthy subjects. Am J Respir Crit Care Med. 1995;152(1):53–8. Epub 1995/07/01. [PubMed]

94. Vlachos-Mayer H, Leigh R, Sharon RF, Hussack P, Hargreave FE. Success and safety of sputum induction in the clinical setting. Eur Respir J. 2000;16(5):997–1000. Epub 2001/01/12. [PubMed]

95. Wong HH, Fahy JV. Safety of one method of sputum induction in asthmatic subjects. Am J Respir Crit Care Med. 1997;156(1):299–303. Epub 1997/07/01. [PubMed]

96. Gibson PG. Use of induced sputum to examine airway inflammation in childhood asthma. J Allergy Clin Immunol. 1998;102(5):S100–1. [PubMed]

97. Hargreave FE. Induced sputum and response to glucocorticoids. J Allergy Clin Immunol. 1998;102(5):S102–5. [PubMed]

98. Green RH, Brightling CE, Woltmann G, Parker D, Wardlaw AJ, Pavord ID. Analysis of induced sputum in adults with asthma: identification of subgroup with isolated sputum neutrophilia and poor response to inhaled corticosteroids. Thorax. 2002;57(10):875–9. Epub 2002/09/27. [PMC free article] [PubMed]

99. Bain BJ. An assessment of the three-population differential count on the Coulter Counter Model S Plus IV. Clin Lab Haematol. 1986;8(4):347–59. Epub 1986/01/01. [PubMed]

100. Ellis A. The pathological anatomy of bronchial asthma. Am J Med Sci. 1908;136:407–28.

101. Schatz M, Wasserman S, Patterson R. The eosinophil and the lung. Arch Intern Med. 1982;142(8):1515–9. Epub 1982/08/01. [PubMed]

102. Frick WE, Sedgwick JB, Busse WW. The appearance of hypodense eosinophils in antigen-dependent late phase asthma. Am Rev Respir Dis. 1989;139(6):1401–6. Epub 1989/06/01. [PubMed]

103. Bousquet J, Chanez P, Lacoste JY, Barneon G, Ghavanian N, Enander I, et al. Eosinophilic inflammation in asthma. N Engl J Med. 1990;323(15):1033–9. Epub 1990/10/11. [PubMed]

104. Ulrik CS. Peripheral eosinophil counts as a marker of disease activity in intrinsic and extrinsic asthma. Clin Exp Allergy. 1995;25(9):820–7. Epub 1995/09/01. [PubMed]

105. Gleich GJ, Adolphson CR. The eosinophilic leukocyte: structure and function. Adv Immunol. 1986;39:177–253. Epub 1986/01/01. [PubMed]

106. Gleich GJ, Adolphson CR, Leiferman KM. The biology of the eosinophilic leukocyte. Annu Rev Med. 1993;44:85–101. Epub 1993/01/01. [PubMed]

107. Jacobsen EA, Taranova AG, Lee NA, Lee JJ. Eosinophils: singularly destructive effector cells or purveyors of immunoregulation? J Allergy Clin Immunol. 2007;119(6):1313–20. Epub 2007/05/08. [PubMed]

108. Flood-Page PT, Menzies-Gow AN, Kay AB, Robinson DS. Eosinophil’s role remains uncertain as anti-interleukin-5 only partially depletes numbers in asthmatic airway. Am J Respir Crit Care Med. 2003;167(2):199–204. Epub 2002/10/31. [PubMed]

109. Wenzel SE. Eosinophils in asthma–closing the loop or opening the door? N Engl J Med. 2009;360(10):1026–8. Epub 2009/03/07. [PubMed]

110. Krause JR, Boggs DR. Search for eosinopenia in hospitalized patients with normal blood leukocyte concentration. Am J Hematol. 1987;24(1):55–63. Epub 1987/01/01. [PubMed]

111. Cunningham AS. Eosinophil counts: age and sex differences. J Pediatr. 1975;87(3):426–7. Epub 1975/09/01. [PubMed]

112. Winkel P, Statland BE, Saunders AM, Osborn H, Kupperman H. Within-day physiologic variation of leukocyte types in healthy subjects as assayed by two automated leukocyte differential analyzers. Am J Clin Pathol. 1981;75(5):693–700. [PubMed]

113. Christensen RD, Hill HR. Exercise-induced changes in the blood concentration of leukocyte populations in teenage athletes. Am J Pediatr Hematol Oncol. 1987;9(2):140–2. Epub 1987/01/01. [PubMed]

114. Mensinga TT, Schouten JP, Weiss ST, Van der Lende R. Relationship of skin test reactivity and eosinophilia to level of pulmonary function in a community-based population study. Am Rev Respir Dis. 1992;146(3):638–43. Epub 1992/09/01. [PubMed]

115. Prussin C, Metcalfe DD. 4. IgE, mast cells, basophils, and eosinophils. J Allergy Clin Immunol. 2003;111(2 Suppl):S486–94. Epub 2003/02/20. [PubMed]

116. Leckie MJ, ten Brinke A, Khan J, Diamant Z, O’Connor BJ, Walls CM, et al. Effects of an interleukin-5 blocking monoclonal antibody on eosinophils, airway hyper-responsiveness, and the late asthmatic response. Lancet. 2000;356(9248):2144–8. Epub 2001/02/24. [PubMed]

117. Nair P, Pizzichini MM, Kjarsgaard M, Inman MD, Efthimiadis A, Pizzichini E, et al. Mepolizumab for prednisone-dependent asthma with sputum eosinophilia. N Engl J Med. 2009;360(10):985–93. Epub 2009/03/07. [PubMed]

118. Massanari M, Holgate ST, Busse WW, Jimenez P, Kianifard F, Zeldin R. Effect of omalizumab on peripheral blood eosinophilia in allergic asthma. Respir Med. 2010;104(2):188–96. [PubMed]

119. Schwartz HJ, Lowell FC, Melby JC. Steroid resistance in bronchial asthma. Ann Intern Med. 1968;69(3):493–9. Epub 1968/09/01. [PubMed]

120. Horn BR, Robin ED, Theodore J, Van Kessel A. Total eosinophil counts in the management of bronchial asthma. N Engl J Med. 1975;292(22):1152–5. Epub 1975/05/29. [PubMed]

121. Taylor KJ, Luksza AR. Peripheral blood eosinophil counts and bronchial responsiveness. Thorax. 1987;42(6):452–6. Epub 1987/06/01. [PMC free article] [PubMed]

122. Bain B, Seed M, Godsland I. Normal values for peripheral blood white cell counts in women of four different ethnic origins. J Clin Pathol. 1984;37(2):188–93. Epub 1984/02/01. [PMC free article] [PubMed]

123. Newman LJ, Platts-Mills TA, Phillips CD, Hazen KC, Gross CW. Chronic sinusitis. Relationship of computed tomographic findings to allergy, asthma, and eosinophilia. JAMA. 1994;271(5):363–7. Epub 1994/02/02. [PubMed]

124. Steinbach KH, Schick P, Trepel F, Raffler H, Dohrmann J, Heilgeist G, et al. Estimation of kinetic parameters of neutrophilic, eosinophilic, and basophilic granulocytes in human blood. Blut. 1979;39(1):27–38. Epub 1979/07/01. [PubMed]

125. Wood LJ, Sehmi R, Dorman S, Hamid Q, Tulic MK, Watson RM, et al. Allergen-induced increases in bone marrow T lymphocytes and interleukin-5 expression in subjects with asthma. Am J Respir Crit Care Med. 2002;166(6):883–9. Epub 2002/09/17. [PubMed]

126. Barr RD, Volaric Z, Koekebakker M. Stimulation of human eosinophilopoiesis by hydrocortisone in vitro. Acta Haematol. 1987;77(1):20–4. [PubMed]

127. Bass DA. Behavior of eosinophil leukocytes in acute inflammation. I. Lack of dependence on adrenal function. J Clin Invest. 1975;55(6):1229–36. Epub 1975/06/01. [PMC free article] [PubMed]

128. Duffield-Lillico AJ, Boyle JO, Zhou XK, Ghosh A, Butala GS, Subbaramaiah K, et al. Levels of prostaglandin E metabolite and leukotriene E(4) are increased in the urine of smokers: evidence that celecoxib shunts arachidonic acid into the 5-lipoxygenase pathway. Cancer Prev Res (Phila) 2009;2(4):322–9. Epub 2009/04/02. [PMC free article] [PubMed]

129. Westcott JY, Maxey KM, MacDonald J, Wenzel SE. Immunoaffinity resin for purification of urinary leukotriene E4. Prostaglandins Other Lipid Mediat. 1998;55(5–6):301–21. Epub 1998/07/08. [PubMed]

130. Daffern PJ, Muilenburg D, Hugli TE, Stevenson DD. Association of urinary leukotriene E4 excretion during aspirin challenges with severity of respiratory responses. J Allergy Clin Immunol. 1999;104(3 Pt 1):559–64. Epub 1999/09/14. [PubMed]

131. O’Shaughnessy KM, Wellings R, Gillies B, Fuller RW. Differential effects of fluticasone propionate on allergen-evoked bronchoconstriction and increased urinary leukotriene E4 excretion. Am Rev Respir Dis. 1993;147(6 Pt 1):1472–6. Epub 1993/06/01. [PubMed]

132. O’Sullivan S, Roquet A, Dahlen B, Dahlen S, Kumlin M. Urinary excretion of inflammatory mediators during allergen-induced early and late phase asthmatic reactions. Clin Exp Allergy. 1998;28(11):1332–9. Epub 1998/11/21. [PubMed]

133. Green SA, Malice MP, Tanaka W, Tozzi CA, Reiss TF. Increase in urinary leukotriene LTE4 levels in acute asthma: correlation with airflow limitation. Thorax. 2004;59(2):100–4. Epub 2004/02/05. [PMC free article] [PubMed]

134. Liu MC, Dube LM, Lancaster J. Acute and chronic effects of a 5-lipoxygenase inhibitor in asthma: a 6-month randomized multicenter trial. Zileuton Study Group. J Allergy Clin Immunol. 1996;98(5 Pt 1):859–71. Epub 1996/11/01. [PubMed]

135. Wenzel SE, Trudeau JB, Kaminsky DA, Cohn J, Martin RJ, Westcott JY. Effect of 5-lipoxygenase inhibition on bronchoconstriction and airway inflammation in nocturnal asthma. Am J Respir Crit Care Med. 1995;152(3):897–905. Epub 1995/09/01. [PubMed]

136. Pavord ID, Ward R, Woltmann G, Wardlaw AJ, Sheller JR, Dworski R. Induced sputum eicosanoid concentrations in asthma. Am J Respir Crit Care Med. 1999;160(6):1905–9. Epub 1999/12/10. [PubMed]

137. Miranda C, Busacker A, Balzar S, Trudeau J, Wenzel SE. Distinguishing severe asthma phenotypes: role of age at onset and eosinophilic inflammation. J Allergy Clin Immunol. 2004;113(1):101–8. Epub 2004/01/10. [PubMed]

138. Christie PE, Tagari P, Ford-Hutchinson AW, Charlesson S, Chee P, Arm JP, et al. Urinary leukotriene E4 concentrations increase after aspirin challenge in aspirin-sensitive asthmatic subjects. Am Rev Respir Dis. 1991;143(5 Pt 1):1025–9. Epub 1991/05/01. [PubMed]

139. Israel E, Fischer AR, Rosenberg MA, Lilly CM, Callery JC, Shapiro J, et al. The pivotal role of 5-lipoxygenase products in the reaction of aspirin-sensitive asthmatics to aspirin. Am Rev Respir Dis. 1993;148(6 Pt 1):1447–51. Epub 1993/12/01. [PubMed]

140. Asano K, Lilly CM, O’Donnell WJ, Israel E, Fischer A, Ransil BJ, et al. Diurnal variation of urinary leukotriene E4 and histamine excretion rates in normal subjects and patients with mild-to-moderate asthma. J Allergy Clin Immunol. 1995;96(5 Pt 1):643–51. Epub 1995/11/01. [PubMed]

141. Rabinovitch N, Zhang L, Gelfand EW. Urine leukotriene E4 levels are associated with decreased pulmonary function in children with persistent airway obstruction. J Allergy Clin Immunol. 2006;118(3):635–40. Epub 2006/09/05. [PubMed]

142. Szefler SJ, Phillips BR, Martinez FD, Chinchilli VM, Lemanske RF, Strunk RC, et al. Characterization of within-subject responses to fluticasone and montelukast in childhood asthma. J Allergy Clin Immunol. 2005;115(2):233–42. Epub 2005/02/08. [PubMed]

143. Micheletto C, Visconti M, Tognella S, Facchini FM, Dal Negro RW. Aspirin induced asthma (AIA) with nasal polyps has the highest basal LTE4 excretion: a study vs AIA without polyps, mild topic asthma, and normal controls. Eur Ann Allergy Clin Immunol. 2006;38(1):20–3. Epub 2006/03/21. [PubMed]

144. Strunk RC, Szefler SJ, Phillips BR, Zeiger RS, Chinchilli VM, Larsen G, et al. Relationship of exhaled nitric oxide to clinical and inflammatory markers of persistent asthma in children. J Allergy Clin Immunol. 2003;112(5):883–92. Epub 2003/11/12. [PubMed]

145. Martin RJ, Szefler SJ, Chinchilli VM, Kraft M, Dolovich M, Boushey HA, et al. Systemic effect comparisons of six inhaled corticosteroid preparations. Am J Respir Crit Care Med. 2002;165(10):1377–83. Epub 2002/05/23. [PubMed]

146. Buyantseva LV, Chinchilli VM, Tulchinsky M, Bascom R, Martin RJ. Alternatives for measuring endogenous adrenocortical activity in asthmatics treated with inhaled corticosteroids. Endocr Res. 2005;31(4):245–58. Epub 2006/01/26. [PubMed]

147. Lewis JG. Steroid analysis in saliva: an overview. Clin Biochem Rev. 2006;27(3):139–46. Epub 2007/02/03. [PMC free article] [PubMed]

148. Buske-Kirschbaum A, von Auer K, Krieger S, Weis S, Rauh W, Hellhammer D. Blunted cortisol responses to psychosocial stress in asthmatic children: a general feature of atopic disease? Psychosom Med. 2003;65(5):806–10. Epub 2003/09/26. [PubMed]

149. Priftis KN, Papadimitriou A, Nicolaidou P, Chrousos GP. Dysregulation of the stress response in asthmatic children. Allergy. 2009;64(1):18–31. Epub 2009/01/10. [PubMed]

150. Whelan GJ, Blumer JL, Martin RJ, Szefler SJ. Fluticasone propionate plasma concentration and systemic effect: effect of delivery device and duration of administration. J Allergy Clin Immunol. 2005;116(3):525–30. Epub 2005/09/15. [PubMed]

151. Kelly HW, Nelson HS. Potential adverse effects of the inhaled corticosteroids. J Allergy Clin Immunol. 2003;112(3):469–79. Epub 2003/09/19. [PubMed]

152. Schauer DA, Linton OW. National Council on Radiation Protection and Measurements report shows substantial medical exposure increase. Radiology. 2009;253(2):293–6. Epub 2009/10/30. [PubMed]

153. Busacker A, Newell JD, Jr, Keefe T, Hoffman EA, Granroth JC, Castro M, et al. A multivariate analysis of risk factors for the air-trapping asthmatic phenotype as measured by quantitative CT analysis. Chest. 2009;135(1):48–56. Epub 2008/08/12. [PMC free article] [PubMed]

154. Vignola AM, Bonanno A, Mirabella A, Riccobono L, Mirabella F, Profita M, et al. Increased levels of elastase and alpha1-antitrypsin in sputum of asthmatic patients. Am J Respir Crit Care Med. 1998;157(2):505–11. Epub 1998/02/26. [PubMed]

155. Wenzel SE, Balzar S, Cundall M, Chu HW. Subepithelial basement membrane immunoreactivity for matrix metalloproteinase 9: association with asthma severity, neutrophilic inflammation, and wound repair. J Allergy Clin Immunol. 2003;111(6):1345–52. Epub 2003/06/06. [PubMed]

156. Fahy JV, Kim KW, Liu J, Boushey HA. Prominent neutrophilic inflammation in sputum from subjects with asthma exacerbation. J Allergy Clin Immunol. 1995;95(4):843–52. Epub 1995/04/01. [PubMed]

157. Chaudhuri R, Livingston E, McMahon AD, Lafferty J, Fraser I, Spears M, et al. Effects of smoking cessation on lung function and airway inflammation in smokers with asthma. Am J Respir Crit Care Med. 2006;174(2):127–33. Epub 2006/04/29. [PubMed]

158. Hastie AT, Moore WC, Meyers DA, Vestal PL, Li H, Peters SP, et al. Analyses of asthma severity phenotypes and inflammatory proteins in subjects stratified by sputum granulocytes. J Allergy Clin Immunol. 2010;125(5):1028–1036. e13. Epub 2010/04/20. [PMC free article] [PubMed]

159. Al-Samri MT, Benedetti A, Prefontaine D, Olivenstein R, Lemiere C, Nair P, et al. Variability of sputum inflammatory cells in asthmatic patients receiving corticosteroid therapy: a prospective study using multiple samples. J Allergy Clin Immunol. 2010;125(5):1161–1163. e4. Epub 2010/04/16. [PubMed]

160. Cowan DC, Cowan JO, Palmay R, Williamson A, Taylor DR. Effects of steroid therapy on inflammatory cell subtypes in asthma. Thorax. 2010;65(5):384–90. Epub 2009/12/10. [PubMed]

161. Paget-Brown AO, Ngamtrakulpanit L, Smith A, Bunyan D, Hom S, Nguyen A, et al. Normative data for pH of exhaled breath condensate. Chest. 2006;129(2):426–30. Epub 2006/02/16. [PubMed]

162. Hunt J, Yu Y, Burns J, Gaston B, Ngamtrakulpanit L, Bunyan D, et al. Identification of acid reflux cough using serial assays of exhaled breath condensate pH. Cough. 2006;2:3. Epub 2006/04/13. [PMC free article] [PubMed]

163. Horvath I. The exhaled biomarker puzzle: bacteria play their card in the exhaled nitric oxide-exhaled breath condensate nitrite game. Thorax. 2005;60(3):179–80. Epub 2005/03/03. [PMC free article] [PubMed]

164. Effros RM. Do low exhaled condensate NH4+ concentrations in asthma reflect reduced pulmonary production? Am J Respir Crit Care Med. 2003;167(1):91. author reply -2. Epub 2002/12/28. [PubMed]

165. National Institutes of Health National Heart, Lung, and Blood Institute. NHLBI Workshop on Oxidative Stress/Inflammation and Heart, Lung, Blood, and Sleep Disorders Meeting Summary; 2004 Aug 18–19; Bethesda (MD): National Institutes of Health, National Heart, Lung, and Blood Institute; 2011. Available from: http://www.nhlbi.nih.gov/meetings/workshops/oxidative-stress.htm.

166. Stepans MB, Wilhelm SL, Rodehorst TK, Smith D, Weinert C. Testing protocols: care of biological samples in a rural setting. Clin Nurs Res. 2009;18(1):6–22. Epub 2009/02/12. [PubMed]

167. Fan H, Hegde PS. The transcriptome in blood: challenges and solutions for robust expression profiling. Curr Mol Med. 2005;5(1):3–10. Epub 2005/02/22. [PubMed]

168. Pieragostino D, Petrucci F, Del Boccio P, Mantini D, Lugaresi A, Tiberio S, et al. Pre-analytical factors in clinical proteomics investigations: impact of ex vivo protein modifications for multiple sclerosis biomarker discovery. J Proteomics. 2010;73(3):579–92. Epub 2009/08/12. [PubMed]

169. Lane AN, Fan TW, Higashi RM. Stable isotope-assisted metabolomics in cancer research. IUBMB Life. 2008;60(2):124–9. Epub 2008/04/02. [PubMed]

170. Slupsky CM, Rankin KN, Wagner J, Fu H, Chang D, Weljie AM, et al. Investigations of the effects of gender, diurnal variation, and age in human urinary metabolomic profiles. Anal Chem. 2007;79(18):6995–7004. Epub 2007/08/19. [PubMed]

171. Hunt J. Exhaled breath condensate: an overview. Immunol Allergy Clin North Am. 2007;27(4):587–96. v. Epub 2007/11/13. [PMC free article] [PubMed]

172. Murugan A, Prys-Picard C, Calhoun WJ. Biomarkers in asthma. Curr Opin Pulm Med. 2009;15(1):12–8. Epub 2008/12/17. [PubMed]

173. Comandini A, Rogliani P, Nunziata A, Cazzola M, Curradi G, Saltini C. Biomarkers of lung damage associated with tobacco smoke in induced sputum. Respir Med. 2009;103(11):1592–613. Epub 2009/07/18. [PubMed]