Influence of urinary creatinine concentration and specific gravity correction in exposure assessment: Comparison...

Urine is a favored matrix for biomonitoring of several important classes of chemicals. Because of variations in concentration, however, urine is often adjusted for dilution. Urinary creatinine (Cr) and specific gravity (SG) have been frequently employed for this purpose. The filtration function of the kidneys may influence the excretion of Cr and urinary SG levels; their influences on urinary exposure biomarker levels are of concern. We hypothesized that dilution adjustment of urine using Cr or SG could influence urinary biomarker levels depending on the kidney function of the subjects. Two groups of adults with different kidney functions, i.e., CKD patients and a general population of Korea, were chosen and measured for Cr, SG, and phthalate metabolites in urine. Urine dilution adjustment could impact the relationship between urinary biomarkers and kidney function differently depending on kidney function of the subjects. In CKD patients, positive associations were observed between eGFR and urinary Cr or SG, while inverse associations were found in the general population. CKD status also influenced the dilution marker-adjusted levels of exposure biomarkers. In CKD patients, Cr and SG correction resulted in higher levels of biomarkers in individuals with worse kidney function, whereas the opposite trend was observed in the general population. The current findings suggest that influences on kidney function should be carefully considered for exposure assessment of urinary chemicals and their association studies, particularly when kidney function is the outcome of concern. Further research is needed to refine urine dilution approaches for evaluating chemical exposure and kidney health outcomes.