In drink driving cases a critical question frequently arises: what was the driver’s blood alcohol concentration not at the time of testing, but at the time of the incident? The answer is provided by the Widmark method – a scientific technique for retrospective calculation of blood alcohol concentration, widely used by forensic experts in Poland and around the world.
What the Widmark Method Is
The Widmark method is a mathematical formula developed by the Swedish pharmacologist Erik M.P. Widmark, allowing blood alcohol concentration to be calculated at any given point in time – both in the past (retrospectively) and in the future (prospectively). The calculation is based on several variables: the amount of alcohol consumed, body weight, sex, the time elapsed since consumption and the individual rate of alcohol elimination.
The Widmark formula in simplified form: alcohol concentration = amount of alcohol consumed / (body weight × distribution coefficient) – (elimination rate × time).
The alcohol distribution coefficient (r) differs between women and men due to differences in body water content: for men it is typically 0.6–0.7, for women 0.5–0.6. The rate of alcohol elimination (β) is typically 0.1–0.2 per mille per hour, with 0.15 per mille per hour most commonly used.
When the Widmark Method Is Used in Court
Calculating the concentration at the time of the incident. A breathalyser or blood test is usually conducted some time after the incident – sometimes ten or twenty minutes later, and in serious accidents even several hours afterwards. The expert retrospectively calculates what concentration the driver had at the time of operating the vehicle.
Verifying the “I drank after the accident” defence. This is one of the most important applications of the method. Where a driver claims to have consumed alcohol only after the incident, the expert checks whether the declared quantity and timing of consumption are consistent with the test result. Where the calculations show that the test result would have been impossible given that quantity and timing, the defence fails.
Establishing the state at the time of driving. At readings close to the legal thresholds (0.10 mg/l or 0.25 mg/l), a retrospective calculation may show that the concentration at the time of driving was below the threshold – which changes the legal classification of the conduct or leads to an acquittal.
Calculating the peak concentration. The expert may also calculate the maximum alcohol concentration in the driver’s system – which is relevant to assessing the degree of social harm of the offence.
Limitations and Uncertainty of the Method
The Widmark method is a scientifically recognised technique, but it is not without limitations. The result is a mathematical calculation – not a measurement. Each variable introduces a degree of uncertainty.
Individual rate of alcohol elimination. The value of β varies between individuals within the range of 0.1–0.2 per mille per hour. The expert typically uses the middle value or the value least favourable to the offender, though the principle of resolving doubts in the defendant’s favour requires the most favourable values to be adopted.
Illness and medication. Liver conditions, prescribed medications and other factors can significantly alter the metabolism of alcohol in a specific individual.
Food and type of alcohol. Consuming alcohol on an empty stomach accelerates absorption, affecting the concentration curve. Different alcoholic drinks are absorbed at different rates.
The absorption phase. Immediately after consumption, alcohol is still being absorbed – concentration may be rising. Calculations are more reliable in the elimination phase (after the peak concentration has been reached).
The Expert’s Opinion in Court Proceedings
A retrospective calculation of blood alcohol concentration using the Widmark method requires the appointment of a forensic toxicologist. The expert’s opinion should contain: the input data for the calculations (test result, time of test, physical data, declarations about consumption), the coefficient values adopted and their justification, the range of calculated concentration (accounting for uncertainty) and conclusions as to the probable concentration at the time of the incident.
A party to the proceedings may challenge the expert’s opinion – requesting clarification, applying for a different expert to be appointed or presenting their own private report. The court is not bound by the expert’s opinion, but if it rejects it, it must give reasons.
Practical Significance for the Defence
Knowledge of the Widmark method and its limitations is critical for an effective defence in drink driving cases.
At borderline readings – a retrospective calculation may show that the concentration at the time of driving was below the criminal offence or misdemeanour threshold.
Where a long time elapsed between the incident and the test – the longer the interval, the greater the uncertainty of the retrospective calculation, which may work in the defendant’s favour.
In a “I drank afterwards” defence – a Widmark analysis can either support or undermine this account. It is worth assessing it before deciding on the defence strategy.
When challenging the expert’s opinion – verifying the coefficient values adopted by the expert and the uncertainty range may reveal errors or assumptions unfavourable to the defendant that can be effectively challenged.
Contact Us
If the calculation of alcohol concentration at the time of the incident plays a key role in your case, or you want to verify an expert’s opinion, it is worth discussing the matter with a lawyer.
It pays to be properly prepared: both in knowing what you can claim and in having a clear strategy.
We handle cases for drivers in Poznań and across the Wielkopolska region, with in-person and online consultations available for clients elsewhere in Poland.
Adwokat Marta Krzyżanowicz and Adwokat Michalina Koligot Law Firm “Adwokat dla Kierowców w Poznaniu” ul. Adama Mickiewicza 18/3, 60-834 Poznań tel. +48 795 001 536 · +48 531 335 713 kontakt@adwokatdlakierowcy.pl · www.adwokatdlakierowcy.pl
This text is for general information purposes only and does not constitute legal advice. Every case requires individual analysis.
