What is a Alcohol Technical Defence?
Alcohol Technical Defence (ATD) calculations are forensic assessments used to estimate a person’s blood alcohol concentration (BAC) at a specific point in time —typically the time of an alleged offence, such as drink driving. These calculations are carried out by trained forensic toxicologists and are often used in court as part of a defence strategy to challenge or clarify the prosecution’s evidence.
ATD calculations take into account various factors, including:
- Volume of alcohol consumed
- Type of alcohol consumed
- Time(s) of consumption
- Body weight, sex, and age
- Time elapsed between drinking and the incident
- Scientific data on alcohol absorption and elimination
Using this data, forensic experts apply established pharmacokinetic models—such as the Widmark formula—to estimate the BAC at the relevant time.
Why are ATDs useful?
Alcohol Technical Defence calculations are a valuable tool in legal proceedings because they provide a scientific basis to interpret or challenge allegations of drink driving. By estimating a person’s blood alcohol concentration (BAC) at the time of an alleged offence, these calculations can demonstrate whether an individual was likely under or over the legal limit when driving. They are especially useful in cases involving post-incident drinking, delays in testing, or inconsistencies in police evidence. These expert assessments offer a clearer understanding of how alcohol may have affected the individual at the relevant time, supporting a fairer evaluation of the case in court.
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* Kingston Analytical Services Toxicology (KAST) is a trading name for Kingston University Enterprises Ltd.
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