Alcohol in forensic medicine: blood levels and their limits
- Topic
Blood alcohol measures ethanol in blood; it helps judge intoxication but must be read with the clinical exam, the sample type and the timing.
What it is
Blood alcohol concentration is the amount of ethanol in the blood. It is requested in injured people, drivers, victims and the dead. Ethanol is absorbed in the stomach and intestine, spreads through body water and is removed mainly by the liver at an almost constant rate. So the figure depends on when the sample was taken, on whether the person was still absorbing or already eliminating, and on the sample type: serum and plasma give higher values than whole blood. After death, bacteria can produce alcohol; vitreous humour and urine help to tell this apart. Intoxication is a clinical diagnosis; the figure supports it, it does not replace it.
Key facts
- The serum/whole-blood ratio is about 1.1 to 1.2 in the literature: serum gives higher figures.
- Breath testing estimates blood alcohol indirectly; the reference test is gas chromatography of blood.
- After death, decomposition, open injuries, hyperglycaemia and time favour alcohol formation.
- Vitreous humour is isolated from gut bacteria: alcohol in vitreous supports drinking in life.
- The metabolites ethyl glucuronide and ethyl sulfate help confirm drinking when the blood result is doubtful.
Blood alcohol grades of the National Traffic Code (art. 152 of Law 769 of 2002, as amended by Law 1696 of 2013), in milligrams of ethanol per hundred millilitres of whole blood
| Grade | Range |
|---|---|
| Grade zero (alcoholemia) | 20 to 39 |
| First grade of intoxication | 40 to 99 |
| Second grade of intoxication | 100 to 149 |
| Third grade of intoxication | 150 and above |
Next step: what to do and document
- Perform and record the clinical intoxication exam (behaviour, speech, gait, coordination, pupils) with the time; do not rely on the figure alone.
- When drawing blood for alcohol in a living person, clean the skin without alcohol and record the exact time.
- State in the report whether the result is from serum, plasma or whole blood, and which method was used.
- After death, send femoral blood, vitreous humour and urine, with preservative and kept cold.
- Hand the sample over under chain of custody; without it the result loses its value as evidence.
- Official text checked on 2026-11-05 at funcionpublica.gov.co.
Legal framework in Colombia
- Law 1696 of 2013, art. 5: amends art. 152 of Law 769 of 2002 and sets the blood alcohol grades and their penalties.
- Law 1696, art. 4: intoxication or blood alcohol is established by a test that causes no injury, as determined by the National Institute of Legal Medicine and Forensic Sciences.
- Law 1696, art. 2: makes negligent homicide more serious if the driver was at grade 1 or above and this was decisive.
- Official text checked on 2026-11-05 at funcionpublica.gov.co (Gestor Normativo).
Test yourself
Think of the answer, then tap the question to see it.
Why is a serum alcohol level not the same as a whole-blood level?
Serum holds more water than whole blood and ethanol distributes in water; so serum gives values about 10 to 20 % higher.
What does alcohol in postmortem blood but none in vitreous humour or urine suggest?
Alcohol produced by bacteria after death, rather than drinking in life.
What range is the first grade of intoxication under Colombian traffic law?
40 to 99 milligrams of ethanol per hundred millilitres of whole blood (art. 152 of Law 769 of 2002, as amended by Law 1696 of 2013).
Is a blood alcohol result enough to diagnose intoxication?
No. Intoxication is a clinical diagnosis; the figure supports the exam, which must be recorded with the time.
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Connections
Educational content on forensic and legal medicine; not legal advice and no substitute for the technical guides of Colombia's National Institute of Legal Medicine and Forensic Sciences. Laws change: each page links the official text and says when it was last checked. For a real case, follow the guide in force and ask the competent authority.
Sources: Ley 1696 de 2013, criminal and administrative penalties for driving under the influence of alcohol or other psychoactive substances (Función Pública, Gestor Normativo, 2013); "Not everything that can be counted counts" in ethanol toxicological results: an antemortem and postmortem technical interpretation focusing on driving under the influence (Forensic Sciences Research, 2024); Vitreous humor analysis for the detection of xenobiotics in forensic toxicology: a review (Forensic Toxicology, 2015).
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