Back to the future: Retrograde alcohol calculations an uncertain science

Retrograde extrapolations, known as back calculations are widely used in forensic toxicology to estimate the blood alcohol concentration of an individual at some prior time. In the UK guidelines have been issued by the United Kingdom and Ireland Association of Forensic Toxicologists) and the Organization of Scientific Area Committees (OSAC) for Forensic Science. However, these guidelines are not fully agreed and open to interpretation. Alcohol elimination rates have been discussed in numerous publications since Widmark’s original data was published. The current guidance from UKIAFT, is to report the most likely back calculated result together with a range of results based on the 95% confidence limit elimination rates (9 to 29 mg/100 mL/hour). The Divisional Court, upheld by the House of Lords, ruled that in order to convict someone for being over the prescribed limit on the basis of any back calculation, the case must be proven beyond reasonable doubt. A 99.73% confidence interval increased to 3 standard deviations at the lower end would provide a greater factual basis for the court and cover a larger proportion of the population, this can be achieved by increasing the elimination range to 8 to 29 mg/100 mL/hour. Retrograde extrapolations also rely on the subject being post absorptive at the prior time. In the UK, back calculations are valid if the subject has not eaten or consumed alcohol within one hour of the back calculation time. Where the subject has eaten prior to the back calculation, experts are instructed to consider whether the back calculation is applicable. In Germany and the United States back calculations are not permitted to a time within 2 h after last drink consumed. The 2 h limit would better meet the highest standard of ‘beyond reasonable doubt’ burden of proof, and should be used. These proposed changes would decrease the uncertainty associated with retrograde calculations carried out by UK toxicologists.