Population ranges for the volume of distribution (Vd ) of alcohol for use in forensic alcohol calculations.

The most accurate method of determining Vd or TBW is either via controlled drinking or isotope dilution studies with an accuracy of ~1.5% [5]. In isotope dilution studies, a known dose of isotope (deuterium/tritium) labeled water is given to the individual. After a known amount of time when the isotope-labeled water has equilibrated with the individual's body water, a blood or saliva sample can be taken and the concentration of the isotope measured. From these data, the individual's TBW can be determined [8]. The second-best method of estimating Vd/TBW is the use of anthropometric equations, with the Watson equation giving an accuracy of ~9.1%–12.8% [6, 7]. In some cases, it is not possible to either experimentally determine Vd/TBW or calculate it, so in this case population data ranges could be used to give the best estimation for Vd/TBW for an individual. This may be as an overall range, by sex, or by subdivision of the sex ranges by body mass index (BMI being a person's weight in kilograms divided by the individual's height in meters squared). It is important to remember that the use of an average or single value for Vd/TBW is not appropriate. To date, there have been no studies in the literature that have presented these data ranges. Using a previously collected data set of TBW measured using isotope dilution we have been able to calculate the Vd of everyone in the study [5] by converting TBW to Vd using Equation 2. These data comprised 1466 individuals of African American, Asian, Caucasian, Hispanic, and Puerto Rican ethnic backgrounds (884 women and 582 men) that had a BMI of between 17 and 80 kg/m2 (women) and 17 and 67 kg/m2 (men) with ages from 18 and 90 years. From these data, the relative frequency distribution of the Vd for males and females was determined and is shown in Figure 1. As expected, due to physiological differences between males and females the median Vd of males (0.71 L/kg) is higher than that of females (0.58 L/kg), although it can be seen there is a large overlap between males and females. The 95% range for males is 0.58–0.83 L/kg and 0.43–0.73 L/kg for females. The 95% range for the entire data set are 0.45–0.81 L/kg. The use of a 95% range gives a better representation of the values of Vd that may be seen in a population as this excludes any potentially extreme values that are not typical of the population. As the BMI of the individuals was recorded in the study, it was possible to look at the 95% range based on Sex and BMI. The BMI ranges were determined according to the World Health Organisation WHO classifications [9]. These data are shown in Figure 2 and Table 1. Using the analysis presented in this study, it is possible, if only the sex of an individual is known, to give suitable ranges for the Vd in forensic alcohol calculations. In transgender individuals undergoing gender-affirming hormone therapy (GAHT) there are alterations in TBW [10] and in this case, if it is known that the individual is receiving GAHT, then the best approach would be to use the 95% range for the entire data set (see Table 1). Although this letter gives Vd ranges that can be used in forensic alcohol calculations it is important to remind readers that outside of experimental studies, the best method of estimating the Vd of an individual is using the Watson et al. equation (see [6, 7]), and this is the recommended approach when the relevant information (age, weight and sex) is available.