New technology can reduce calculation times when determining the degree of uncertainty in blood alcohol concentration estimations.

Expert toxicologists are frequently faced with questions involving estimations of blood alcohol levels that occurred during a variety of forensic situations, particularly those involving the operation of a vehicle while intoxicated (DUI). The most popular formula for estimating an individual’s blood alcohol concentration (BAC) was published by Widmark in 1932.¹ The formula is fairly complex when calculated and takes into account alcohol distribution and elimination in humans, but fails to account for the degree of uncertainty (error) inherent in those two processes as well as the process of alcohol absorption altogether. The judicial system has evolved since Widmark’s pioneering work in estimating BAC and has held the forensic discipline to new and higher standards for the admission and testimony of scientific evidence. In order to maintain objectivity, these new standards require experts to account for and report the degree of uncertainty when using theoretical calculations for legal purposes.¹ Determination of the degree of uncertainty, however, is time consuming and requires a number of additional calculations which toxicologists must perform for each particular case. Recently, commercial software has been designed to overcome this problem and assist toxicologists in accounting for such uncertainties, allowing for the derivation of objective, unbiased conclusions in a much more timely fashion. This reduces the need for hand calculations and increases the number of cases that can be worked in laboratories which may be understaffed as a result of the current economic climate.

Alcohol metabolism consists of three key processes (absorption, distribution, elimination) which occur simultaneously and affect how much alcohol is able to accumulate in the blood stream and which is ultimately responsible for causing intoxication of an individual. Alcohol absorption is the first process to occur and continues until all of the ingested alcohol has entered the blood stream. As soon as the alcohol has been absorbed it immediately begins its distribution into the body tissues and fluids as well as undergoing elimination by the oxidation systems in the liver.

Empirical studies on the rate of alcohol absorption published in 2005 reveal that alcohol is typically fully absorbed into the blood stream in about 0.5 to 2 hours² and have resulted in the publication of a formula for the purpose of estimating BAC during the absorptive phase.³ The speed of absorption is highly variable and dependent upon a number of factors including the gastrointestinal contents (i.e. food), the speed of consumption, the concentration of the beverage, and other factors.⁴ The first order rate constant for the absorption of alcohol has been found to be 2.5–6.5 (+/-1.5) with the higher numbers corresponding to a faster rate of absorption.³ Due to the intraindividual variation of the rate of alcohol absorption caused by a number of different factors that are typically unquantifiable in a practical sense, the use of two extreme rates of absorption will account for these uncertainties by providing a narrow range of BAC values at a particular time of interest.