Improving the specific gravity adjustment method for assessing urinary concentrations of toxic substances.

Changes in urinary flow induce changes in urinary concentrations of toxic substances. The authors modified the conventional specific gravity adjustment method for measuring urinary concentration of toxic substances to compensate for the dilution effects from varying degrees of hydration. The conventional specific gravity adjustment method is a special case to the more general method proposed in this article. The conventional method generally does not correct the urinary concentration to the mean specific gravity of urine. It requires the assumption that a change in urine flow preserves the relative ratio between the mass of the xenobiotic and the mass of total dissolved solids. The derivation of the modified specific gravity adjustment method shows that a change in urine flow does not necessarily preserve this ratio. An experimental slope between urinary flow and urinary specific gravity was linear on a log scale. A ratio was formed between the experimental slope and Araki's "b" slopes for various substances to predict changes in urine concentrations due to changes in the specific gravity of a spot sample. Since excretion rates typically vary for different substances with changes in urinary flow, an appropriately weighted exponential adjustment factor is required for each substance to normalize its concentration to the standard specific gravity of urine.