Back-estimation of blood alcohol concentration.

The recent paper by Al-lanqawi et al. (1992) deserves attention because of the medicolegal implications of their work, e.g. in connection with the prosecution of drinking drivers. The back-estimation of a motorist's blood alcohol concentration (BAC) is a controversial subject (Jones, 1988). Some forensic scientists condone this practice whereas others consider it highly speculative. The statutory limit of alcohol permitted in a motorist's blood, such as 80 mg dl-1 (800 mg l-l) in UK, refers to the concentration of ethanol measured in a specimen of whole blood and not plasma (Jones, 1992). In the study reported by Al-lanqawi et al. (1992), gas chromatography was used to measure the concentrations of ethanol in plasma and the rate of disappearance of ethanol (k0), assuming a linear post-peak decay phase, was 186 ± 26 mg I1h-1 (mean ± s.d.). But plasma contains more water and therefore more ethanol than an equal volume of whole blood and the mean plasma:whole blood ratio of ethanol concentration is about 1.10:1 (Jones et al., 1990). This roughly 10% difference in concentration of ethanol means that the slope of the plasma-ethanol disappearance phase will be steeper than the whole-blood disappearance phase (Jones et al., 1992). Note that the concentrations of ethanol in plasma and whole blood must reach the zero level at the same time. This difference in k0 depending on the specimen analysed, becomes important when Al-lanqawi et al. (1992) compare their average rate of ethanol elimination (186 mg I1h-1) with the presumed value for the population (150 mg 1-1 h-1). This latter figure, cited from the book by Walls & Brownlie (1985), was derived from studies of ethanol pharmacokinetics after obtaining and analysing whole blood (Widmark, 1932). When making back estimations of BAC for medicolegal purposes, the calculations are strongly biased in favour of the suspect-this gives the benefit of the doubt to likely slow metabolisers. It would be unthinkable to use the fastest rate of ethanol elimination reported by Al-lanqawi et al. (1992) of 238 mg F-1 h-1 or even the presumed average rate of 150 mg 1-1 h-1. In my experience, a rate of ethanol disappearance of 80 to 100 mg 1-1 h-1 is more realistic if and when backestimations must be made. Interestingly, using these more conservative estimates of the rate of ethanol elimination, very few if any of the subjects investigated by Al-lanqawi et al. (1992) would have had their BAC overestimated at 1 h post-drinking. The pharmacokinetic parameters Cmax, CO and k0 determined by Al-lanqawi et al. (1992) had coefficients of variation of 15.5%, 11.2% and 13.9% respectively. This amount of variability is not excessive compared with human pharmacokinetics of many other drugs. The much larger coefficient of variation (41%) for the tmax is not surprising because the time to peak BAC is a discrete variable having fixed values of 0.25, 0.5, 0.75, 1 h etc. Two of the volunteer subjects began the experiment with a BAC of 10-20 mg 1-1 which suggests they might have been drinking the night before. I wonder if these two individuals also had the fastest rates of ethanol elimination? It is widely known that the microsomal pathway of ethanol metabolism is boosted after prolonged exposure to substrate. Al-lanqawi et al. (1992) do not reflect on the most appropriate pharmacokinetic model to describe the disposition of ethanol. Although a linear decay profile is clearly evident in the concentration-time profiles this does not verify that zero-order kinetics are operating. It appears that the slope of the linear segment increases with an increase in the dose administered (Jones, 1993). In conclusion, Al-lanqawi et al. (1992) contribute a useful study of the pharmacokinetics of ethanol based on analysis of plasma specimens. However, the kinetic parameters they derived are not the same as those for kinetics in whole blood, in particular ko is about 10% higher for plasma specimens. This should be borne in mind when contemplating making back-estimations of BAC or when ethanol measurements at hospital clinical laboratories are reported in drunk driving trials (Jones, 1992). The existing notions about making back estimations of BAC based on some presumed value of ko are flawed because a mulitude of other factors influence this variable besides the rate of enzymatic oxidation of ethanol.