Temporal variability in urinary concentrations of perchlorate, nitrate, thiocyanate and iodide among children.

Perchlorate, nitrate and thiocyanate are ubiquitous in the environment, and human exposure to these chemicals is accurately measured in urine. Biomarkers of these chemicals represent a person's recent exposure, however, little is known on the temporal variability of the use of a single measurement of these biomarkers. Healthy Hispanic and Black children (6-10-year-old) donated urine samples over 6 months. To assess temporal variability, we used three statistical methods (n=29; 153 urine samples): intraclass correlation coefficient (ICC), Spearman's correlation coefficient between concentrations measured at different timepoints and surrogate category analysis to assess how well tertile ranking by a single biomarker measurement represented the average concentration over 6 months. The ICC measure of reproducibility was poor (0.10-0.12) for perchlorate, nitrate and iodide; and fair for thiocyanate (0.36). The correlations for each biomarker across multiple sampling times ranged from 0.01-0.57. Surrogate analysis showed consistent results for almost every surrogate tertile. Results demonstrate fair temporal reliability in the spot urine concentrations of the three NIS inhibitors and iodide. Surrogate analysis show that single-spot urine samples reliably categorize participant's exposure providing support for the use of a single sample as an exposure measure in epidemiological studies that use relative ranking of exposure.