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Is there a problem using forensic alcohol calculations for transgender individuals?

Written by: Alan Baker 13th October, 2022

Transgender is defined as people who have a gender identity which differs from the sex they were assigned at birth. Around the world there is estimated to be around 2% of the population who are transgender.

Currently forensic alcohol calculations are the most performed forensic calculation. Forensic alcohol calculations require the knowledge of the sex of the individual.

However, this may disadvantage transgender people as there is research currently undergoing which has demonstrated that there are physiological changes in individuals undergoing gender-affirming hormonal treatment (GAHT), which is used to treat gender dysphoria.  Current data shows that around 80% of transgender individuals are either taking or plan to take GAHT.

GAHT transgender women commonly receive oestrogen, often in conjunction with an androgen blocker or gonadotrophin-releasing hormone analogues. GAHT transgender men receive testosterone.

These treatments are known to alter body characteristics and therefore these changes may influence the results of forensic alcohol calculations that are often based on the sex of an individual rather than their gender.

As far as we are aware, there are no guidelines or recommendations that consider the body changes that occur in individuals undergoing GAHT.

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:


Co – the hypothetical BAC at time zero before any metabolism has occurred (mg/100 ml).

A – amount of pure ethanol consumed (g).

Fwater – fraction of blood volume that is water (% w/v).

TBW – total body water of an individual (L).


In the case of forensic alcohol calculations, the two variables that are likely to be altered by GAHT therapy are TBW and Fwater.

There have been several studies looking at the changes of both body fat and fat-free mass in individuals undergoing GAHT.

The studies have shown that, following the beginning of GAHT in transgender women, there is an increase in body weight, an increase in body fat, and a decrease in lean body mass.

Compared to transgender men where there is a decrease in body weight, decrease in body fat, and an increase in lean body mass following the commencement of GAHT.


Mean percentage differences in total body water between cisgender men and women and the percentage change in total body water in transgender people following gender-affirming hormone therapy.


The important variable for forensic alcohol calculations is lean body mass. Lean body mass is proportional to TBW, as the water content of the tissues is considered a constant. So, if the changes of lean body mass following GAHT are known, the changes in TWB after GAHT can be estimated.

The studied TBW in transgender individuals can then be applied in forensic alcohol calculations.

This study has confirmed that transgender individuals who have undergone 12 months of GAHT are currently disadvantaged using the present cisgender TBW equations.

It is recommended that if it is not known if the individual is cisgender or transgender 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.