Reliability of Spot Urine Samples

Abstract

P-144 Introduction: The extent to which spot urine samples yield consistent, stable, and uniform results indicating over-exposure to chemicals in the workplace is often debated. Both the Occupational Safety and Health Administration (OSHA) and the American Conference of Governmental Industrial Hygienists (ACGIH) have established levels of urinary metabolites above which there is concern about over-exposure to an airborne chemical. At times, spot urine samples are also used when conducting biomonitoring studies of contaminants to which persons in the community are exposed (e.g., low level exposure). This paper describes one of the difficulties in characterizing urine data from a person exposed to benzene, toluene and xylene. Methods: A voluntary study was conducted where a worker was exposed to low levels of benzene, toluene, and xylene for three days. The 8-hr time weighted average (TWA) airborne concentration on each day of the study was less than 1 ppm (range 0.1–0.83 ppm). Twenty-nine urine samples were collected during three days of the study and compared to thirty background samples. The concentration of t,t-muconic acid (ttMa) and phenol were measured. Results: Approximately 7% of the background samples had concentrations of phenol over the OSHA emergency benzene standard and 30% had concentrations of ttMA over the ACGIH biological exposure index (BEI). Statistically, phenol corrected for specific gravity was significantly greater in the urine samples collected during the exposure period than in the urine collected during the background period (p=0.0002). However, the concentrations of uncorrected phenol, phenol corrected for creatinine, uncorrected ttMA, and ttMA corrected for creatinine were not statistically different from background concentrations for this volunteer (p=0.50, 0.39, 0.32, 0.48, respectively). Discussion and Conclusions: These results support the belief that spot urine samples are rarely reliable indicators of exposure to airborne toxicants in workers or citizens unless numerous samples are collected over several days when exposure occurred, as well as on days when no exposure occurred (e.g., background). Multiple samples are necessary because diet, metabolic differences, and intra-day variability in the concentrations of most urinary metabolites in any individual vary considerably. With respect to benzene, if 8-hr TWA airborne concentrations do not exceed 1 ppm spot urine samples are unlikely to be informative. In our experience, only when a unique metabolite or other biomarker is being studied in a person (e.g., not present in the background), and pharmacokinetic characteristics are considered, is spot urine sampling likely to be useful.