In medico-legal cases involving alcohol, the Widmark formula is used to back-calculate blood alcohol concentration (BAC) at the time of driving. If a person has a current BAC of 0.06 g/dL, weighs 70 kg (r-factor = 0.68), consumed alcohol 3 hours ago, and metabolism rate is 0.015 g/dL/hour, what was the BAC at the time of alleged driving (3 hours ago)?
- A 0.06 g/dL
- B 0.105 g/dL ✓
- C 0.045 g/dL
- D 0.075 g/dL
Explanation
Back-calculation: BAC at time of driving = current BAC + (metabolism rate × hours elapsed). BAC at driving = 0.06 + (0.015 × 3) = 0.06 + 0.045 = 0.105 g/dL. In India, the legal BAC limit for driving is 0.03 g/dL (30 mg/100 mL); in many countries it is 0.08 g/dL. This individual at 0.105 g/dL would have been above the legal limit at the time of driving. The Widmark r-factor (volume of distribution for ethanol) is used to estimate total body alcohol from dose calculations, while back-calculation from a measured BAC primarily uses the elimination rate. This type of calculation has important medico-legal implications in drunk driving prosecutions.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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Written and medically reviewed by the StethoPrep medical team.