Evaluation of physiological measures for correcting variation in urinary output: Implications for assessing environmental chemical exposure in children.

Urinary contaminant concentrations are commonly adjusted by creatinine to account for the variability in urinary output. This approach may not be optimal among children due to developmental growth of muscle mass and the associated increase in creatinine formation. An alternative approach is to measure the specific gravity of the urine sample, which reflects the solute concentration of the urine. We compare the appropriateness of urinary creatinine and urinary-specific gravity as factors for correcting morning and evening spot urine samples collected from 23 children (3-11 years) for a total of 41 days in four different seasons. Two linear mixed-effects models were fit using age, sex, season, and sample collection time (morning/evening) as predictors with specific gravity and creatinine as dependent variables. Specific gravity was significantly associated with the sample collection time (P<0.001) with morning samples higher than evening samples. Creatinine was significantly associated with season (P<0.05), sample collection time (P<0.0001), and age (P<0.0001). Creatinine levels were higher during the summer compared to the other seasons, higher in the morning compared to the evening, and higher with increases in children's age. Normalizing the children's spot urine samples using creatinine would introduce bias to the data analysis. Whereas using specific gravity to correct for variable urinary output would be more robust. In addition, measuring specific gravity is relatively easy, does not require the use of chemicals, and the results are available instantaneously.