The second part of this review describes the principles and practice of forensic congener analysis as an alternative way to evaluate claims of drinking alcohol after driving. Congener analysis was developed, perfected and practised in Germany as a way to evaluate hip-flask defences. This kind of defence challenge arises frequently when the drunk driving suspect is not apprehended at the wheel and especially after hit-and-run incidents. Besides ethanol and water, alcoholic beverages contain trace amounts of many other low-molecular substances, known collectively as the congeners, which impart the characteristic smell and taste to the drink. Importantly, the congener profile can be used to identify a particular kind of alcoholic beverage. Forensic congener analysis entails making a qualitative and quantitative analysis of ethanol, methanol, n-propanol and the isomers of butanol in blood and urine from the apprehended driver and comparing the results with the known congener profile of the alcoholic beverage allegedly consumed after driving. Interpreting the results of congener analysis requires knowledge about the absorption, distribution and elimination pattern of the congener alcohols, including their oxidation and conjugation reactions, and any metabolic interactions with ethanol. Complications arise if drinks with widely different congener profiles are consumed or if the same beverage was ingested both before and after driving. Despite these limitations, congener analysis can furnish compelling evidence to challenge or support claims of drinking alcohol after driving.
The failure of medical-psychological examination to provide convincing recommendations concerning the regranting of licences in a significant number of cases illustrates the need for objective laboratory testing. Experience in Germany shows that blood-alcohol concentration alone could lead to misleading recommendations, and suggests that laboratory testing is best done on samples taken at the time of the offence, rather than subsequently at medical-psychological investigation.
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