Adjustment for urinary flow rate: an improved approach to biological monitoring.

The use of urinary monitoring in medical surveillance programs in industry requires the development of an accurate and unbiased index of urinary concentrations of occupational toxins. To examine the effects of urinary flow rate on adjusted and non-adjusted urinary excretion of 11 heavy metals and organic substances, 19 metal-foundry workers were studied during four time periods of the day under conditions of water restriction and loading. The results indicate that urinary flow rate significantly affects not only the non-adjusted urinary concentration for all substances, but also affects timed excretion as well as concentrations adjusted to urinary specific gravity and to urinary creatinine during any time period of the day. On the other hand, the concentration adjusted to urinary flow rate (UF-adjusted concentration) is independent by definition of urinary flow; therefore, this adjustment is applicable for highly diluted and highly concentrated urine samples without repetition of urine collection. It is concluded that UF-adjusted concentration is a useful index for the measurement of most urinary substances, while adjustments to urinary specific gravity and to urinary creatinine concentration have only limited utility in evaluating toxin levels in spot urine samples.