Forensic alcohol calculations in transgender individuals undergoing gender‐affirming hormonal treatment

Abstract There are an increasing number of individuals undergoing gender‐affirming hormonal treatment (GAHT) to treat gender dysphoria. Current forensic alcohol calculations require knowledge of the sex of the individual, but this may disadvantage trans people as research has demonstrated that there are physiological changes in individuals who are undergoing GAHT. Using previously published studies on total body water (TBW) in cis individuals, and the known changes in lean body mass and hematocrit in trans individuals, it is possible to estimate TBW in trans individuals and compare them to those cis equation estimations. When using these revised rubrics, we determined that for trans women the use of the cis male anthropometric TBW equation only gives a small underestimation of TBW (0.9%) compared to the underestimation of TBW using the cis female TBW equation (−17.7%). For trans men, the use of the cis female TBW equation gives the largest disadvantage, underestimating TBW by −10.8% compared to the cis male TBW equation, that overestimates TBW by 6.6%. For this reason, we recommend that if the sex at birth of an individual is not known or disclosed, any forensic alcohol calculations in a forensic alcohol reports are made assuming that the gender declared by the individual is their sex at birth. Further research to develop validated anthropometric TBW equations are urgently needed as to not disadvantage trans people when forensic alcohol calculations are carried out.


| INTRODUC TI ON
Around the world there are estimated to be between 0.1% and 2% of the population who are transgender [1]. Transgender is defined as people who have a gender identity which differs from the sex they were assigned at birth (sex at birth) [1]. The antonym of transgender is cisgender which describes a person whose sex at birth and gender identity are the same. The true number of transgender individuals around the world is unclear and is most likely being underestimated due to cultural sensitivities [2]. Based on current data, around 80% of transgender individuals are either taking or want to take genderaffirming hormone therapy (GAHT) [3]. The use of GAHT aims to align the characteristics of an individual with their gender identity.
GAHT transgender women commonly receive estrogen, often in conjunction with an androgen blocker or gonadotrophin-releasing hormone analogues. GAHT transgender men receive testosterone [4,5]. These treatments are known to alter the body characteristics of the individuals taking them [4,5], and these body changes may influence the results of forensic alcohol calculations that are often based on the sex of an individual rather than their gender. In forensic science, it is important to have a rigorous evidence base for forensic practices, particularly making sure that the practices do not disadvantage individuals or groups of individuals that may lead to miscarriages of justice [6]. Forensic alcohol calculations, probably the most performed forensic calculations, have a solid evidence base due to many years of research (summarized in Ref. [7]). However, as far as the authors are aware there are no published guidelines, or recommendations, for forensic alcohol calculations that take into account the body changes that occur in individuals undergoing GAHT. The United Kingdom Association of Forensic Toxicologists (UKIAFT) alcohol calculation guidelines do state that the information collected for forensic alcohol calculations should include "sex at birth" [8]. The assumption in these guidelines are that the individual undergoing GAHT will have a total body water similar to individuals of the sex they were assigned at birth. However, to date there are no studies looking at the body changes in transgender individuals with regard to forensic alcohol calculations. Additionally, depending on the legal jurisdiction, if an individual has legally changed their gender, they are under no obligation to disclose their sex at birth. The aim of this study is to investigate forensic alcohol calculations in individuals that are undergoing GAHT.

| Forensic alcohol calculations
The most common form of the equation, known as the Widmark equation, to estimate the blood alcohol concentration of an individual after consumption of a known amount of alcohol is: C o -the hypothetical BAC at time zero before any metabolism has occurred (mg/100 ml). A -amount of pure ethanol consumed (g). F water -fraction of blood volume that is water (% w/v). TBW -total body water of an individual (L).
For an individual undergoing GAHT, there will be various physiological changes to their body. In the case of forensic alcohol calculations, the two variables that are likely to be altered by GAHT therapy are TBW and F water .

| Total body water (TBW) and gender-affirming hormonal treatment (GAHT)
There have been a number of studies looking at the changes of both body fat and fat-free mass in individuals undergoing GAHT. These studies have shown that on average, following the commencement of GAHT in trans women, there is an increase in body weight, an increase in body fat, and a decrease in lean body mass [9]. On average, in trans men, there is a decrease in body weight, decrease in body fat, and an increase in lean body mass following the commencement of GAHT [9]. The variable of importance here for forensic alcohol calculations is that of lean body mass. Lean body mass is proportional to TBW, as the water content of the tissues is considered a constant [10,11]. Thus, if the changes of lean body mass following GAHT are known, the changes in TWB after GAHT can be estimated. The re- to −2%; 95% CI] in trans women and increased by +10% [9%-11%; 95% CI] in trans men [12]. Thus, the mean change in TBW for trans men would be approximately +10% and approximately −3% in trans women ( Figure 1).

| Percentage of blood that is water (F water ) and gender-affirming hormonal treatment (GAHT)
Forensic alcohol calculations to determine the blood alcohol concentration of an individual after consuming a known amount of ethanol rely on not only the estimation of the individuals TBW, but also F water . Taking into account any potential changes in the

| Changes in total body water (TBW) in individuals undergoing gender-affirming hormone therapy (GAHT)
The studies above have shown that there are body changes that will alter the results of forensic alcohol calculations in transgender individuals when compared to cisgender individuals. The work of Klaver et al [12] demonstrated that on average, the mean change in TBW for trans men was approximately +10% and approximately −3% in trans women. As can be seen in Table 1 the actual change in lean body mass differs according to the body mass index (BMI) of the individual before the start of GAHT.
In order to investigate the effects of these changes on the  TA B L E 3 Estimated mean total body water (TBW) of transgender individuals after 12 months of gender-affirming hormonal treatment (GAHT) using data from Maskell et al [22] assuming that the GAHT caused changes to TBW according to data collected by Klaver et al [12] Trans women (TBW (L)) Trans men (TBW (L))  [18,19]. These data sets are comprised of the sex at birth, height, body mass (weight), age, and total body water (measured by the 3 H-dilution method). Table 2 shows the mean (±SD) total body water ranges grouped into BMI of both cis men and cis women. Based on these data, cis males have a mean TBW of 45.9 ± 7.4 L (n = 582) and cis women have a mean TBW of 33.9 ± 6.2 L (n = 884) with a mean difference of 35.4% (~12 L). The measured TBW of the cis males and cis females was revised to estimate the TBW of trans individuals based on percentage changes in lean body mass based on the BMI of the cis individuals, detailed in Table 1. As can be seen in Table 3 the mean (±SD) TBW of trans women based on the starting cis male population is estimated to be 44.3 ± 7.0 L and for trans men to be 37.3 ± 6.4 L based on a starting cis female population. The only current guidance that applies to alcohol calculations in trans individuals suggests that the gender at birth should be used for ethanol calculations [8]. However, based on the data from Klaver et al [12], on average the TBW of a trans woman would be 3% lower than a cis man and 10% higher for a cis woman compared to a trans man. Overall, as shown in Table 3, there is a +30.7%   ABV vodka) or 10 UK units of ethanol 80 g (~1 × 750 ml bottle of wine, 13% ABV). Table 5 shows the calculated C o , with Table 6 showing the mean and percentage differences in C o between the various groups. As with the TBW the estima-   For this reason, we recommend that if the sex at birth of an individual is not known or not disclosed any forensic alcohol calculations in the report are made assuming that the gender declared by the individual is their sex at birth.

| Ethanol elimination rates
The rate of ethanol elimination is also an important parameter in forensic alcohol calculations [7]. However as the same ethanol elimination rates and ranges are used for both sexes in forensic ethanol calculations trans specific ethanol elimination rates and ranges do not need to be used [21].

| Limitations
This study is based on the mean changes that occur in TBW and

| CON CLUS IONS
This study has demonstrated that transgender individuals that have undergone 12 months of GAHT are currently disadvantaged by the use of the present cis TBW equations. We recommend that if it is not known if the individual is cis gender or trans gender then a forensic alcohol calculation report should state the assumption that the gender given by the individual is considered to be the sex at birth.
Further research to develop validated anthropometric TBW equations are urgently needed as to not disadvantage trans people when forensic alcohol calculations are carried out.

ACK N OWLED G M ENTS
The authors would like to thank Fiona Flaherty for her advice on rubrics, Jennifer Limoges for her constructive comments on the draft manuscript and Caitlyn Maskell for her help with the figure.

D EFI N ITI O N S
Sex at birth -(sex assigned at birth). Sex at birth is typically assigned based on a person's reproductive system and other physical characteristics.
Cisgender -describes a person whose gender identity and sex assigned at birth are the same.
Transgender -a person who has a gender identity or gender expression that differs from the sex that they were assigned at birth.