Intra- and inter-individual variability of urinary phthalate metabolite concentrations in Hmong women of reproductive age.

The reproducibility of urinary phthalate metabolite concentrations has not been well characterized in non-pregnant women of reproductive age. Our primary study objectives were to describe the distribution of urinary phthalate metabolites concentrations among a population of Hmong women of reproductive age, and to evaluate intra- and inter-individual variability of phthalate metabolite concentrations. Ten phthalate metabolites were measured in first-morning urine samples collected from 45 women and 20 of their spouses, who were members of the Fox River Environment and Diet Study cohort in Green Bay, Wisconsin. Repeated first-morning urine samples were collected and analyzed from 25 women, who provided up to three samples over approximately 1 month. Measurement variability was assessed using intraclass correlations (ICCs) and surrogate category analysis. Linear mixed models were used to evaluate the associations between participant characteristics and phthalate metabolite concentrations. Nine of the 10 phthalate metabolites were detected in >80% of all analyzed samples, of which seven were detected in all samples. As a measure of reliability, ICCs were strongest for monobenzyl phthalate (0.64) and weakest for the metabolites of di(2-ethylhexyl)phthalate (DEHP) (ranging from 0.13 to 0.22). Similarly, surrogate category analysis suggested that a single urine sample characterized an average 1-month exposure with reasonable accuracy across low, medium and high tertiles for all metabolites, except the DEHP metabolites. Geometric mean concentrations of monoethyl phthalate increased with age, but patterns by education, income, body mass index, environmental tobacco smoke or season were not observed when measures were adjusted for urinary dilution. Our results suggest that the participant characteristics assessed in this study have limited influence on inter-individual variability of phthalate metabolite concentrations. With regard to intra-individual variability, our results suggest that urinary concentrations of some phthalate metabolites are more reproducible over time and are less subjected to exposure misclassification than others (e.g., metabolites of DEHP).