Replacing creatinine measurements with specific gravity values to adjust urine cotinine concentrations.

Creatinine and specific gravity (relative density) measurements both allow differences in urine concentration to be taken into account in determining urine cotinine concentrations. In this study we demonstrate that the variance of urine cotinine measurements is reduced comparably when either creatinine or specific gravity measurements are used for correction. This reduction in variability improves the correlation between urine cotinine measurements and clinical endpoints. In this study, the clinical endpoints were pulmonary function in a population of nonsmoking children with asthma, 42% of whom were reported to have been exposed to environmental tobacco smoke. When corrected by either creatinine measurements or specific gravity values, the urine cotinine measurements performed as well or better than reported exposure (and comparably with each other) in assessments of lung function. A dose-response relationship was also more consistently apparent. Specific gravity values can be used reliably in place of creatinine values to adjust urine cotinine measurements for both research and clinical purposes.