Reference limits for urine/blood ratios of ethanol in two successive voids from drinking drivers

Specimens of venous whole blood and two successive urinary voids were collected from 450 individuals apprehended for driving under the influence of alcohol in Sweden. The first specimen of urine (UAC-1) was obtained as soon as possible after arrest, and the second void (UAC-2) was collected about 60 min later (mean 66 min, range 30-130). A specimen of venous blood was drawn approximately 30 min after the first urine sample was collected. Ethanol was determined in blood and urine by headspace gas chromatography, a method with high analytical precision (coefficient of variation approximately 1%). The mean UAC for the first void was 2.60 g/L (range 0.21-5.35) compared with 2.40 g/L (range 0.16-5.50) in the second void. The mean concentration of alcohol in venous blood (BAC) was 1.97 g/L (range 0.08-4.57). The concentrations of ethanol in the two voids of urine were highly correlated (r = 0.97, residual standard deviation [SD] 0.22 g/L). The UAC and BAC results were also highly correlated; r = 0.958 (residual SD 0.28 g/L) for the first void and r = 0.978 (residual SD 0.21 g/L) for the second void. The concentration of ethanol in the first void (UAC-1) was higher than the second void (UAC-2) in 383 (87%) instances, decreasing by 0.23 g/L/h on average. In 57 instances (13%), UAC-1 was less or equal to UAC-2 with a mean increase of 0.19 g/L. When BAC exceeded 0.5 g/L (N = 429), the mean UAC-1/BAC ratio was 1.345 with 95% reference limits of 0.968 and 1.72, which agreed well with median (2.5th and 97.5th percentiles) of 1.325 (0.938 and 1.79). For the second void, the mean UAC-2/BAC ratio was 1.221 with 95% reference limits of 0.988 and 1.45 and with a median (2.5th and 97.5th percentiles) of 1.226 (0.997 and 1.46). These reference limits are appropriate to use when a person's venous BAC needs to be estimated with reasonable scientific certainty from the concentration determined in specimens of urine.