Urine specimens are commonly employed in cystic fibrosis (CF) biomarker research for the measurement of infectious and inflammatory mediators. Urine is an ideal specimen, given it can be collected non-invasively, is usually plentiful and poses minimal risk. Spot urine samples are preferred to 24-hour collections for feasibility and to avoid improper and incomplete 24-hour collection. The major disadvantage of spot urine samples is the variation in dilution effects, sample volume and the rate of urine production (1
). In an attempt to adjust for this variation, urinary creatinine (UCr) concentration is most commonly used in a ratio format to normalize analyte quantification for specimen concentration. The normalization process involves dividing the concentration of the analyte of interest by the UCr concentration obtained in the same urine sample, with the result reported as the concentration of target analyte per milligram of creatinine.
Creatinine is a waste product of muscle metabolism and is excreted in urine at a relatively constant rate through glomerular filtration (2
). It is the relative stability within an individual that makes it an attractive approach for normalization of analyte concentrations. However, the rate of creatinine excretion has been shown to vary across different patient demographics, introducing the possibility of bias in urinary biomarker measurements that use UCr for normalization (4
). Creatinine excretion has been documented to increase throughout the growth period, peaking at 20–30 years of age, and decreasing thereafter, reflecting the changes in body muscle tissue mass (1
). Furthermore, UCr concentrations are known to vary by gender, ethnicity and dietary protein levels (5
). Creatinine adjustment is thought to work best when the renal elimination mechanism of the analyte is similar to the renal elimination mechanism of creatinine (2
Patients with CF may have reduced creatinine levels or irregular urinary excretion of creatinine due to several biological factors, including reduced muscle mass related to nutritional difficulties, the presence of a hyper-catabolic state from chronic systemic inflammation, limited exercise capacity, and frequent pulmonary exacerbations or illness (10
). CF patients may also have renal impairment (decreased glomerular filtration rate), potentially related to cumulative aminoglycoside use, even in the presence of a normal blood urea and serum creatinine (10
). Given the potential variability in excretion and quantity, UCr may not be an appropriate normalizing factor to use for urinary biomarker quantification in a CF population during illness or when making comparisons with this population.
An alternative method for normalization is specific gravity (SG), which is defined as the ratio of the density of a urine specimen to the density of water. SG values increase with solute concentration. Our aim was to investigate the effect of using UCr as a normalization factor for spot urine samples in hospitalized CF patients and for comparison with samples obtained from patients with asthma and from healthy controls without lung disease. To provide further evaluation of the appropriate measure of urinary concentration, SG values will also be examined. This investigation is necessary if accurate inferences from urinary biomarker studies within the hospitalized CF population are to be achieved.