Forensic toxicology: which samples and how
- Procedure
To learn whether a substance caused or contributed to a death or a crime, the right samples are taken, clearly labelled and kept cold.
What it is
Forensic toxicology looks for alcohol, medicines, drugs and poisons in samples from the living or the dead. A result is only useful if the right sample is taken from the right site, stored properly and kept under chain of custody. In the body, the reference sample is peripheral blood (femoral vein), because blood from the heart and body cavities changes after death (postmortem redistribution). Vitreous humour, urine, gastric contents and hair complete the work: each answers a different question. In the living (for example, suspected drug-facilitated assault), urine and blood are collected as early as possible.
What to document
- The exact site of each sample (for example, "right femoral vein blood"), not just "blood".
- Known or estimated time of death, time of sampling and the interval between them.
- State of the body: decomposition, burns, open injuries, refrigeration.
- Treatment received: fluids, transfusions, resuscitation drugs, sedation.
- The specific clinical suspicion (which substance and why) to guide the laboratory.
- Every person who held the sample, with the date and time of each handover.
Structure
- Peripheral blood (femoral or iliac): the reference for measuring drugs and alcohol after death.
- Heart blood: useful to detect (what is present), unreliable to quantify (how much).
- Vitreous humour: alcohol, glucose, urea, electrolytes; useful when blood is altered or missing.
- Urine: longer detection window than blood; shows exposure, not effect at the time.
- Gastric contents: recent ingestion by mouth; collected whole, with the volume recorded.
- Head hair, cut close to the scalp: chronic use or remote exposure.
- Liver and other tissues: when there is no blood (decomposed or burnt bodies).
How it is done
- Before sampling, read the clinical record and note the drugs given during treatment and resuscitation.
- Keep hospital admission samples (blood, urine) taken before death: they are the most valuable.
- At autopsy, draw femoral blood separately from heart blood and label each tube with its source.
- Collect vitreous humour from both eyes, urine and all gastric contents, each in its own container.
- Use tubes with preservative (sodium fluoride) and anticoagulant for blood; close each container tightly.
- Label: case name or code, sample, anatomical site, date, time and who took it.
- Refrigerate at once; freeze if analysis is delayed, as the laboratory advises.
- Package, seal and hand over with the chain-of-custody form to the judicial police.
Toxicology samples after death and what they are for
| Sample | What it is for | Main limit |
|---|---|---|
| Femoral blood | Measuring drugs and alcohol | May be missing or altered in decomposed bodies |
| Heart blood | Detecting which substances are present | Postmortem redistribution: falsely high levels |
| Vitreous humour | Alcohol, glucose, electrolytes | Small volume; little data for many drugs |
| Urine | Exposure in the previous days | Does not show the effect at the time of death |
| Gastric contents | Recent oral ingestion | Contaminates nearby blood by diffusion |
| Hair | Use over weeks or months | Useless for exposure in the last hours |
Next step: what to do and document
- If a patient dies after a poisoning or a violent injury, do not discard admission samples: keep them cold and notify the authorities.
- Record in the chart the drugs given, with times, so the toxicologist does not mistake them for the cause.
- Request analysis with a clear question: what is suspected and what needs to be known.
- Hand samples over only with the chain-of-custody form completed and signed.
- Law 906 of 2004 (art. 255) makes health staff who come into contact with evidence responsible for collecting, preserving and handing it to the authorities.
- Official text checked on 2026-11-05 at funcionpublica.gov.co.
Common mistakes
- Taking "blood" from the chest cavity without saying so: the result cannot be interpreted.
- Mixing blood from several sites in one tube.
- Using gel-separator tubes for toxicology: some absorb drugs.
- Cleaning the skin with alcohol before drawing blood for a blood alcohol test in the living.
- Leaving samples at room temperature or unsealed while waiting for the police.
Test yourself
Think of the answer, then tap the question to see it.
Which blood sample is preferred to measure drug levels after death?
Peripheral blood from the femoral (or iliac) vein, because it changes less through postmortem redistribution than heart or body-cavity blood.
Why is vitreous humour useful in postmortem toxicology?
It is isolated inside the eye and far from gut bacteria, so it undergoes less putrefaction and redistribution; it is used mainly for alcohol, glucose and electrolytes.
What does hair analysis show that blood does not?
Exposure or use over weeks or months (and, by segment, when it happened); it cannot show what was in the body in the last hours.
Which samples from a patient who dies in the emergency department should be kept?
The blood and urine taken on admission, ideally before treatment: they reflect the state in life. Keep them cold and hand them over under chain of custody.
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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: Complete republication: National Association of Medical Examiners position paper: Recommendations for the investigation, diagnosis, and certification of deaths related to opioid drugs (Journal of Medical Toxicology, 2014); Vitreous humor analysis for the detection of xenobiotics in forensic toxicology: a review (Forensic Toxicology, 2015); Postmortem redistribution of drugs: a literature review (Forensic Science, Medicine and Pathology, 2024); Ley 906 de 2004, Código de Procedimiento Penal (Code of Criminal Procedure) (Función Pública, Gestor Normativo, 2004).
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