Estimating blood alcohol from urinary alcohol at autopsy

Urine alcohol concentration (UAC) and blood alcohol concentration (BAC) measured by gas chromatography were available from 435 medicolegal autopsies. Simple linear regression with BAC as outcome variable and UAC as predictor variable (range, 3-587 mg%) gave the regression equation BAC = -5.6 + 0.811UAC with 95% prediction interval +/- 0.026 square root of [9465804 + (UAC-213.3)2] and 99% prediction interval +/- 0.034 square root of [9465804 + (UAC-213.3)2]. The standard error of the slope was 0.013 and the 95% confidence interval for the slope 0.785-0.837. In practice, a BAC of 80 mg% is predicted with 95% certainty by a UAC of 204 mg% and similarly a BAC of 150% by a UAC of 291 mg%. The prediction interval is too wide to be helpful in the assessment of an individual case fatality. The UAC is useful in corroborating but not in predicting BAC.