## Characteristic Odour in Cyanide Poisoning **Key Point:** The **bitter almond odour** is the classic forensic sign of cyanide poisoning, though it is not always present and its absence does NOT rule out cyanide toxicity. ### Why Bitter Almond Odour? The odour comes from hydrogen cyanide (HCN) gas released from cyanide salts (potassium cyanide, sodium cyanide) when exposed to moisture, gastric acid, or decomposition. The volatile HCN has a characteristic bitter almond smell. ### Important Limitations 1. **Genetic variation:** Approximately 20–40% of the population cannot detect the odour due to anosmia (genetic inability to smell cyanide) 2. **Absence does not exclude cyanide:** Many cases of cyanide poisoning show no detectable odour 3. **Decomposition:** In advanced decomposition, the odour may be masked by other putrefactive odours 4. **Concentration-dependent:** Very low concentrations may not produce a detectable smell **High-Yield:** The bitter almond odour is a **supportive but not diagnostic** finding. Forensic diagnosis requires toxicological analysis (blood cyanide levels, urine thiocyanate). **Clinical Pearl:** Never rely solely on odour for cyanide diagnosis — it is a historical observation that may assist in initial suspicion but must be confirmed by laboratory testing. ### Differential Odours in Poisoning | Poison | Odour | |--------|-------| | Cyanide | Bitter almond | | Hydrogen sulphide | Rotten egg | | Organophosphates | Garlic | | Acetone/Isopropanol | Fruity/acetone | | Corrosive acids | Pungent, acrid |
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