Clinical Assessment and Diagnosis of the Poisoned Patient
This domain tests structured bedside reasoning: can you take a truncated history, extract the discriminating physical findings, order the right test at the right time, and resist over-reading a screening immunoassay? The exam favours pattern recognition disciplined by physiology - identifying a toxidrome and then explaining which findings do and do not fit.
History and examination
Establish time of ingestion, amount, co-ingestants, formulation (sustained release), intent, and whether others may have been exposed. Vitals first: bradycardia with hypotension suggests sympatholytic or cholinergic excess; tachycardia with hypertension suggests sympathomimetic or anticholinergic. Then pupils, skin (dry versus diaphoretic), mucous membranes, bowel sounds, and a deliberate search for trauma and needle marks.
The classic toxidromes
- Cholinergic: salivation, lacrimation, urination, diarrhoea, bronchorrhoea, bradycardia, miosis, fasciculations.
- Anticholinergic: dry flushed skin, mydriasis, delirium, urinary retention, decreased bowel sounds.
- Sympathomimetic: diaphoresis, mydriasis, tachycardia, hypertension, agitation.
- Opioid: miosis, respiratory depression, decreased consciousness.
- Sedative-hypnotic: depressed consciousness with relatively normal vitals early.
- Serotonergic: clonus, hyperreflexia, agitation, hyperthermia.
Targeted investigations
ECG for QRS duration and QT interval, blood gas, electrolytes with anion and osmolal gaps, renal and hepatic function, co-oximetry when carboxyhaemoglobin or methaemoglobin is possible, and specific quantitative levels drawn at the correct time (acetaminophen and salicylate in every intentional overdose). Immunoassays are screening tools with cross-reactivity, not confirmation.
Worked example. Sodium 140, chloride 100, bicarbonate 10 mmol/L gives an anion gap of 30. Measured osmolality 320; calculated 2 × 140 + glucose/18 + urea/2.8 = 280 + 5.5 + 10 ≈ 295, giving an osmolal gap of about 25. A high anion-gap metabolic acidosis with a raised osmolal gap points to a toxic alcohol - methanol or ethylene glycol - before lactate or ketoacidosis, and treatment with an alcohol dehydrogenase blocker should start while confirmatory testing is pending.
Traps that cost marks
- Relying on a negative urine screen to exclude ingestion.
- Attributing a raised anion gap to lactate without measuring it, when an unmeasured osmole is present.
- A normal pulse oximetry reading in carbon monoxide poisoning.
- Interpreting an acetaminophen level drawn before four hours after a single acute ingestion.