Objectives: We characterized the within- and between-child variability of urinary organophosphorus and pyrethroid metabolites in 23 participants of the Children's Pesticide Exposure Study Washington over 1 year and examined the ability of one to four spot urine samples to categorize mean exposures.
Although our study differed from the animal studies because we measured adult and not gestational exposure, our findings suggesting that a single urine sample, used to categorize a subject's exposure, did not adequately measure 3-month average exposure to MEHP.
To determine the predictive ability of a single urine sample to correctly categorize a subject's exposure into high, medium, or low tertiles, we calculated actual values (mean and geometric mean values) for surrogate categories.
The surrogate category analysis indicated that when a single sample was used to categorize exposure into quartiles, the quartile mean values increased monotonically only 14 51% of the time (Table 3).
This implies that in statistical analyses in which only a single urine sample is available to categorize a subject's 3-month exposure to MEHP, there is likely exposure measure misclassification resulting in bias toward the null hypothesis for exposure response relationships.
Geometric mean ACR was determined from two consecutive first-morning urine samples and used to categorize subjects as follows: normoalbuminuria (ACR <30 mg/g), microalbuminuria (ACR 30 299 mg/g), and macroalbuminuria (ACR ≥300 mg/g) (7).
A participatory wealth-ranking approach was used to categorize sample households into various wealth categories.
Ten subjects were presented with pictures of familiar and unfamiliar objects (see Figure 1 for some sample pictures) and asked to categorize them, while EEG signals were recorded from 128 channels and stored for offline analysis.
Similarly, results from another study conducted among a group of 35 children suggest that BPA concentrations in a single urine sample can be used to categorize the 6-month average exposure to BPA (Teitelbaum et al. 2007).
This information was used to categorize samples into a second scheme consisting of conventional samples from companies with (n = 59) and without (n = 46) stated policies prohibiting arsenical use.
