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Follow the sample through the laboratory
- Collection and chain of custody: who handled the sample and when.
- Specimens: femoral blood, urine, vitreous humour, hair, gastric contents and liver, and what each tells you.
- Preservation: sodium fluoride for alcohol samples; refrigeration or freezing for most drugs.
- Extraction: liquid–liquid extraction at the right pH, solid-phase extraction, hydrolysis of glucuronides.
- Screening: colour tests, TLC and immunoassay.
- Confirmation: GC-MS and LC-MS/MS with internal standards and identification criteria.
Drugs of abuse seen in Pakistan
Know heroin (and its marker 6-MAM), charas and other cannabis products (THC and THC-COOH), methamphetamine or ice (and its precursors ephedrine and P2P), cocaine (benzoylecgonine and cocaethylene), tramadol, codeine syrups and benzodiazepines. For each, learn the plant or precursor source, a presumptive test and the confirmatory approach.
Poisons that come up again and again
- Organophosphates: cholinesterase activity, atropine and pralidoxime.
- Aluminium phosphide: phosphine, silver nitrate paper test, no specific antidote.
- Paraquat: sodium dithionite test, pulmonary fibrosis.
- Carbon monoxide and cyanide: carboxyhaemoglobin, bright red lividity, antidotes.
- Arsenic, lead and mercury: Reinsch and Marsh tests, chelators.
- Datura: atropine and hyoscine, anticholinergic toxidrome.
Interpretation is examinable
Questions increasingly test interpretation rather than recall: post-mortem redistribution, why vitreous alcohol matters in decomposed bodies, why poppy seeds can give a positive opiate screen, and why an immunoassay positive must be confirmed. Practise these as short scenarios.
Quality and the law
Learn ISO/IEC 17025, proficiency testing, method validation terms (bias, precision, LOD, LOQ, carry-over, matrix effects) and measurement uncertainty. On the legal side, know the purpose of the Control of Narcotic Substances Act 1997, the role of the Anti-Narcotics Force and the three UN drug control conventions.