Toxicology · Short notes

Forensic Toxicology: Short Notes & Cheat Sheet

How drugs and poisons are detected and interpreted for legal purposes: specimens, chain of custody, screening, confirmation and post-mortem pitfalls.

Short notes

Read these before you attempt the topic's MCQs.

Each step is documented so the result can be defended in court; confirmation always uses a different principle from screening.

Post-mortem toxicology: did a drug or poison cause or contribute to death?
Human performance toxicology: impairment in driving, assault or other offences.
Workplace and drug-facilitated crime testing, where cut-offs and chain of custody are critical.
Results must be defensible in court, so every step is documented and methods are validated (ISO/IEC 17025).

Femoral (peripheral) blood is preferred after death; heart blood is affected by post-mortem redistribution.
Urine concentrates drugs and metabolites, so it shows exposure, not impairment.
Vitreous humour resists putrefaction; it is used for alcohol, glucose and electrolytes.
Hair records exposure over weeks to months (about 1 cm per month), allowing segmental analysis.
Gastric contents and liver help in oral poisonings and when blood is unavailable.
Blood for alcohol is preserved with sodium fluoride and an anticoagulant such as potassium oxalate.

Presumptive tests (colour tests, TLC, immunoassay) are fast but can cross-react.
Confirmation uses a different principle: GC-MS (EI at 70 eV, library match) or LC-MS/MS (MRM transitions).
Deuterated internal standards correct for extraction losses and matrix effects.
Basic drugs are extracted after making the sample alkaline; glucuronides are hydrolysed first with β-glucuronidase.
Reports state measurement uncertainty for quantitative results such as blood alcohol.

Post-mortem redistribution raises central blood levels of lipophilic drugs with a large Vd (tricyclics, digoxin).
Ethanol can be produced after death by microbes; compare blood with vitreous and urine.
Tolerance means opioid users may survive levels fatal to naive people.
Poppy seeds give morphine and codeine but not 6-MAM.

High-yield facts in one place. Cover the right-hand column and test yourself.

Marker	Meaning
6-Monoacetylmorphine (6-MAM)	Recent heroin use
Benzoylecgonine	Cocaine use
Cocaethylene	Cocaine taken with alcohol
THC-COOH	Cannabis use (days to weeks)
Ethyl glucuronide (EtG)	Recent or chronic alcohol intake
Carboxyhaemoglobin	Carbon monoxide exposure
Low cholinesterase activity	Organophosphate or carbamate exposure

Specimen	Best use
Femoral blood	Quantitation after death
Urine	Screening; detection window of days
Vitreous humour	Alcohol, electrolytes, glucose
Hair	History over months
Gastric contents	Recently swallowed poison
Liver	Drugs concentrated by metabolism

Poison	Test
Aluminium phosphide	Silver nitrate paper turns black (phosphine)
Paraquat	Sodium dithionite turns urine blue
Arsenic	Reinsch and Marsh tests
Carbon monoxide	CO-oximetry for COHb
Ethanol	Headspace GC-FID

Femoral for Facts: Use femoral blood for post-mortem quantitation; heart blood is inflated by redistribution.
Screen, then Seal it with MS: Every presumptive positive is confirmed by a mass spectrometric method before reporting.

Attempt the Forensic Toxicology MCQs while the notes are fresh. Anything you get wrong is saved to your incorrect list for the next revision round.