Fifty-seven children with asthma (19 female) participated in the study (; available at www.jpeds.com
), all of whom completed the ACQ6 and provided urine specimens at the initial study encounter, defined as baseline. Ages ranged between 6 and 20 years with a median age of 11 years. On average, children in the study sample fell in higher percentiles of body mass index (BMI) than children in the general population, with a median BMI percentile of 73.1 and IQR of 48.2% to 97.5%. Forty-five participants were able to perform spirometry with adequate technique, 22 of whom performed repeat spirometry after administration of albuterol. Median spirometric parameters fell within normal limits, as defined by NIH guidelines,2
specifically the percent of the predicted value of the forced expiratory volume in 1 second (FEV1
percent predicted, median 96.6% predicted), the ratio of the forced expiratory volume in 1 second to the forced vital capacity (FEV1
/FVC, median 0.85), and the percent change in FEV1
after administration of bronchodilator (median 6.4%). Many participants (70%) were treated with inhaled corticosteroids at baseline. Three of the subjects were also concomitantly maintained on oral corticosteroids at baseline. A significant proportion of participants (64%) were atopic on the basis of skin testing results. The median (IQR) BrTyr level was 0.12 (0.00–0.31) ng/mg creatinine; the median (IQR) NO level was 9.1 (6.0–19.8) parts per billion (ppb). Median (IQR) ACQ6 scores at baseline and follow-up were 0.50 (0.17–1.08) and 0.33 (0.17–1.50), respectively, consistent with adequately controlled asthma.21
At follow-up 6 weeks after baseline, 36% of participants had experienced an asthma exacerbation since baseline, including all three of the participants maintained on oral corticosteroids. Features of the study participants and symptoms associated with asthma exacerbations are summarized in .
Features of study participants (n = 57)
Urinary BrTyr and exhaled NO levels did not differ between males and females, who had median (IQR) levels of BrTyr of 0.07 (0.00–0.21) ng/mg and 0.25 (0.00–0.53) ng/mg creatinine, respectively, P = .12, and median (IQR) levels of NO of 8.4 (5.4–16.4) ppb and 11.0 (6.7–31.9) ppb, respectively, P = .27. NO, but not BrTyr, correlated directly with age (R = 0.38, P = .007); neither correlated with BMI percentile. Urinary BrTyr levels demonstrated no significant correlation with spirometric parameters; NO correlated inversely with FEV1/FVC (R = −0.32, P = .04), but did not correlate with FEV1 percent predicted or the percent change in FEV1 after bronchodilator. Neither BrTyr nor NO correlated with any of the complete blood count parameters, including percent eosinophils, or IgE levels. BrTyr and NO did not correlate with one another ().
Correlations between markers and clinical parameters
NO, but not BrTyr, was higher in participants with positive allergy skin test results (atopic) than those with negative allergy skin test results (nonatopic). The median (IQR) NO level among atopic participants was 11.1 (6.1–25.9) ppb versus 6.3 (3.9–11.5) ppb among nonatopic participants (P = .03. The median (IQR) BrTyr level among atopic participants was 0.14 (0.00–0.47) ng/mg creatinine versus 0.00 (0.00–0.47) ng/mg creatinine among nonatopics, P = .39.
Urinary BrTyr levels demonstrated a striking and consistent direct correlation with ACQ6 scores at baseline (R = 0.38, P = .004) and follow-up (R = 0.39, P = .008) (). In contrast, NO did not (R = 0.20, P = .17 at baseline; R = 0.27, P = .10 at follow-up). BrTyr further correlated directly with each of the six individual items on the ACQ6 at baseline and all but one (β2-agonist use) of the items on the ACQ6 at follow-up. NO, on the other hand, did not correlate significantly with any of the individual items on the ACQ6 at baseline; at follow-up, NO correlated directly with only one item (activity limitation, R = 0.41, P = .01). Interestingly, eosinophil counts, in contrast to BrTyr levels, did not correlate with either baseline (R = 0.03, P = .88) or follow-up (R = 0.27, P = .26) ACQ6 scores.
We next examined whether baseline levels of either urinary BrTyr or exhaled breath NO were elevated among the subset of subjects who experienced an asthma exacerbation over the ensuing 6 weeks of follow-up versus those who did not experience an exacerbation. BrTyr levels were significantly higher among those destined to experience a near-term exacerbation (BrTyr median [IQR] 0.00 [0.00–0.18] vs 0.23 [0.02–0.68]; P
= .02). In contrast, baseline levels of exhaled breath NO were comparable among those who experienced an exacerbation and those who did not (median [IQR] NO levels of 12.0 [5.7–16.9] vs 9.1 [6.7–22.9]; P
= .71). To further assess the capacity of urinary BrTyr and exhaled NO to predict asthma exacerbations and indexes of asthma control, BrTyr and NO were dichotomized into high and low levels. High levels were defined as greater than their respective median levels; low levels were defined as less than or equal to their respective median levels. FEV1
percent predicted, FEV1
/FVC, and percent change in FEV1
after bronchodilator were also dichotomized by their respective medians. ACQ6 scores at baseline and follow-up were dichotomized as ≥1.5 versus <1.5, a cut point that previous research has deemed optimal to detect inadequately controlled asthma (positive predictive value of 0.87).21
Individual ACQ6 item scores were dichotomized as at least one versus zero, indicating that the participant, in the week preceding questionnaire administration, has woken up at night because of asthma, experienced asthma symptoms on waking in the morning, was limited in his/her activities because of asthma, experienced shortness of breath, wheezed, or used a short-acting β2
-agonist. Finally, the presence of an asthma exacerbation between baseline and follow-up was assessed as previously defined.
A high level of BrTyr at baseline conferred 18.1-fold the odds of inadequately controlled asthma at baseline, as defined by an ACQ6 score ≥1.5, than a low level of BrTyr (95% CI of 2.1–153.1, P = .0004). Moreover, participants with high levels of BrTyr were more likely to experience morning symptoms (OR, 95% CI of 3.0, 1.0–9.1; P = .04), activity limitation (4.7, 1.5–14.5; P = .005), shortness of breath (3.2, 1.0–9.9; P = .04), and wheezing (3.3, 1.0–10.7; P = .04) at baseline, as well as use a short-acting β2-agonist (4.2, 1.3–13.0; P = .01) at baseline. High levels of BrTyr showed a trend toward experiencing nocturnal waking at baseline, but this failed to reach statistical significance (2.7, 0.8–9.2; P = .11) (). Unlike BrTyr, a high level of NO at baseline did not confer significantly greater odds of inadequately controlled asthma at baseline, neither with respect to the composite ACQ6 score nor to any of the individual asthma symptom items ().
Figure 1 ORs and 95% CI for the associations between high levels of A, bromotyrosine and B, nitric oxide (B) and uncontrolled asthma at baseline. Results shown represent the ORs (filled circles) and 95% CI (lines) of having uncontrolled asthma versus controlled (more ...)
Similarly, a high level of BrTyr at baseline conferred 5.0-fold the odds of inadequately controlled asthma at follow-up than a low level of BrTyr (95% CI 1.1–22.2, P = .02). Participants with high levels of BrTyr were also more likely to experience morning symptoms (OR, 95% CI of 3.8, 1.1–13.5; P = .03), activity limitation (4.6, 1.3–16.3; P = .02), and wheezing (4.3, 1.1–17.1; P = .03) at follow-up. High levels of baseline BrTyr showed a trend toward experiencing nocturnal waking (5.3, 0.9–29.1; P = .06) and shortness of breath (3.0, 0.9–10.4; P = .07). They were not, however, more likely to use a short-acting β2-agonist at follow-up (). Again, unlike BrTyr, participants with high levels of NO at baseline were not significantly more likely to have inadequately controlled asthma at follow-up, neither with respect to the composite ACQ6 score nor to any of the individual items ().
Figure 2 ORs and 95% CI for the associations between high levels of A, bromotyrosine and B, nitric oxide and uncontrolled asthma at follow-up. Results shown represent the ORs (filled circles) and 95% CI (lines) of having uncontrolled asthma versus controlled asthma (more ...)
Urinary BrTyr, exhaled NO, asthma control as monitored by ACQ6, and traditional asthma laboratory diagnostics were next examined for their ability to predict asthma exacerbation over the ensuing 6-week interval. Remarkably, of all blood, urine, and spirometric measures examined, only a high level of BrTyr predicted the presence of an asthma exacerbation by follow-up; participants with elevated BrTyr levels demonstrated a 4.0-fold increased likelihood of experiencing an exacerbation by follow-up (95% CI 1.1–14.7, P = .03) compared with subjects with low BrTyr levels ().
Figure 3 ORs and 95% CI for the associations between the presence of an asthma exacerbation by follow-up and increased airway obstruction, greater airway reversibility, ACQ6 scores, high levels of bromotyrosine, and high levels of nitric oxide at baseline. Results (more ...)
Interestingly, the median BrTyr level for females was more than triple the median BrTyr level for males. Among the 28 participants with high BrTyr levels, 12 (43%) were female and 16 (57%) were male; among the 29 participants with low BrTyr levels, 7 (24%) were female, and 22 (76%) were male in the low group (P = .13). To ensure that our findings were not influenced by a sex effect, we performed post-hoc logistic regression analyses to adjust for sex. After adjusting for sex, participants with high levels of BrTyr had 16.6-fold the odds of uncontrolled asthma at baseline (95% CI 2.8–319.7, P = .01), as defined by a baseline ACQ6 score > 1.5; 4.9-fold the odds of uncontrolled asthma at follow-up (95% CI 1.2–26.4, P = .04), as defined by a follow-up ACQ6 score > 1.5; and 4.0-fold the odds of an asthma exacerbation by follow-up (95% CI 1.1–16.6, P = .05). These analyses demonstrate that our findings are robust to sex variability.