Estimation of blood alcohol concentrations after social drinking.

Requests for estimates of blood alcohol concentrations (BACs) are often made when blood samples are taken some hours after the time of interest. Many believe that such estimates are not reliable because the subject's alcohol clearance rate is never known and often there is uncertainty as to whether the subject was postabsorptive at the time in question. In order to evaluate the potential errors associated with BAC estimates under these non-ideal conditions, BAC estimates were compared with empirical data obtained from 24 healthy males, ranging in age from 22 to 56 years, who took part in a three hour social drinking session. One blood sample for alcohol analysis was taken from each subject approximately 1 hour after drinking stopped and another was taken approximately 3.5 hours after drinking stopped. Estimations of BACs at the blood sampling time points were made assuming each person had a constant blood alcohol clearance rate in the range of 10 to 20 mg/dL/h (0.01 to 0.02 g/dL/h) over the whole of the experimental period. A variety of methods were used to estimate the volume of distribution for alcohol. All BAC estimations were made assuming complete absorption and full equilibration of the total alcohol dose. The results showed that actual BACs were usually within or very close to the range of "forward" estimates based on the known alcohol doses. Furthermore, most BACs measured about an hour after cessation of drinking were within or very close to the predicted range based on back extrapolation from the actual 3.5 hour BAC result.