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.

Alcohol absorption is an important process affecting the BAC at a particular time. However, most legal cases involving alcohol involve BAC estimations and determinations in the post-absorption phase. Because of this, more attention needs to be directed to the remaining two processes involved in alcohol metabolism—distribution and elimination. Alcohol distribution does not occur uniformly throughout the body. The distribution of alcohol throughout the body (blood, tissues, organs, fluids) is a complicated and highly variable process. The distribution of alcohol begins as soon as the alcohol is absorbed into the bloodstream and is a significant factor that affects how much alcohol remains in the blood stream causing intoxication. The Widmark formula is the first and most widely used formula for estimating BAC, utilizing a mean distribution coefficient of 0.68 for males and 0.55 for females.1 The use of a mean distribution coefficient to account for such a highly variable process carries the potential of a biased (erroneous) result depending on the degree to which Widmark’s study sample represents the general population. Research has indicated that the mean distribution values originally published by Widmark in 1932 tend to differ significantly from other empirically derived distribution calculations.1,2 The use of a single theoretical calculation based on the Widmark coefficient alone, which relies on a mean value algebraically derived from a small scale study in 1932, fails to accurately account for the interand intra-individual variability inherent in the general population.1 Five notable, independently derived distribution calculations have been published using various methods of accounting for alcohol distribution in the human body.1,2 These five distribution coefficients rely on a number of different anthropological factors serving to account for uncertainties arising from inter-individual variations in alcohol distribution. These five distribution coefficients are furthermore able to account for uncertainties arising from intra-individual variations in alcohol distribution when used together.2 This is because when each of the five distribution calculations are compared to each other they tend to agree with one another and have higher values than Widmark’s mean distribution coefficient for both males and females to a statistically significant degree.1 Keeping in mind that the distribution coefficient and BAC value are inversely related to each other, the Widmark formula usually results in higher estimations of BAC while the other five refinements typically agree with one another.2 The use of all six independently derived distribution coefficients together will provide the toxicologist with a more objective, unbiased opinion of the estimated BAC at a particular time because of their ability to account for uncertainties stemming from both inter- and intraindividual variations in alcohol distribution (Figure 1).

estimated blood alcohol concentration chart

The third key process of alcohol metabolism is elimination. Alcohol elimination occurs as soon as the alcohol is absorbed into the blood stream and continues until all of the alcohol is eliminated from the body. Alcohol is eliminated by two oxidation systems located in the liver—the alcohol dehydrogenase enzyme system (ADH) and the cytochrome P450 dependent ethanol oxidizing system (CYP2E1).4 Once the ADH system becomes saturated, alcohol is eliminated at a zero order rate. However, at higher concentrations of alcohol the CYP2E1 is induced and contributes to alcohol elimination at a first order rate.4 Because alcohol elimination is a combination of both the ADH and CYP2E1 systems and therefore is a combination of zero order and first order rate kinetics, one single rate of elimination is not sufficient to account for the uncertainty caused by intra-individual variations in the speed of elimination during a forensic estimation. Empirical data show that rates of alcohol elimination for most social drinkers fall within a range of 0.01g/dl/hr to 0.024g/dl/hr with an average rate ranging between 0.015g/dl/hr to 0.018g/dl/hr.1,2 While the rate of alcohol elimination has been widely accepted to fall between these ranges, the rate is not static throughout a drinking scenario because of the number of factors affecting the efficiency of the oxidation systems responsible for eliminating the alcohol.4 The dynamic nature of alcohol elimination requires the use of two extreme rates in order to fully account for these uncertainties in alcohol elimination by providing a narrow range of BAC values at a particular time of interest.

When providing expert opinions regarding an estimated BAC value at a particular time of interest it is important for the toxicologist to utilize all relevant scientific information in order to provide an unbiased, objective opinion. Because of the complexity of alcohol metabolism and the requirement of forensic toxicologists to account for any and all uncertainties inherent in this dynamic process the calculations can become very time consuming and complex when calculated by hand. New technology (BAC Tracker®), however, reduces the time requirement of hand calculations and allows experts to consistently provide unbiased estimations of BAC that instantaneously account for the degree of uncertainty by utilizing all relevant scientific information.What is particularly notable about this software is that it is designed in an understandable format for both experts and laymen alike. The user-friendliness of this software allows for all parties involved in alcohol litigation to have open access to a fundamental understanding of the principal factors affecting alcohol metabolism: scientists and attorneys, prosecution and defense.


  1. Zuba, D. & Piekoszewski W. Uncertainty in Theoretical Calculations of Alcohol Concentration. Presented at the 17th International Conference on Alcohol, Drugs and Traffic Safety (ICADTS), Glasgow, UK, 2004. Available at:
  2. Posey, D. & Mozayani, A. The Estimation of Blood Alcohol Concentration: Widmark Revisited. For. Sci. Med. Path. 2007; 3:33- 39.
  3. Uemura, K., Fujimiya, T., Ohbora, Y., Yashuhara, M., Yoshida, K. Individual Differences in the Kinetics of Alcohol Absorption and Elimination. A Human Study. For. Sci. Med. Path. 2005; 1:24-7.
  4. Casarett and Doull’s Toxicology: The Basic Science of Poisons, 6th ed.; Klaassen, C., Ed.; McGraw-Hill: New York, pp 107-237, (2001).


Henry Swofford can be reached at BAC Tracker, 2921 Lenox Rd. NE, Suite 411, Atlanta, GA 30324,