## Confirmatory Investigation for Cyanide Poisoning ### Why Serum Cyanide Level is the Gold Standard **Key Point:** Serum cyanide level measured by spectrophotometry or high-performance liquid chromatography (HPLC) is the most specific and direct confirmatory test for cyanide poisoning. **High-Yield:** Serum cyanide levels: - Normal: < 0.2 μmol/L - Toxic: > 1 μmol/L (symptomatic poisoning typically occurs at levels > 3 μmol/L) - Fatal: > 10 μmol/L ### Timing and Specimen Collection 1. Blood sample must be collected in a heparinized tube (not EDTA) to prevent interference 2. Sample should be analyzed within 24 hours as cyanide is volatile 3. Immediate preservation in ice is recommended 4. Results may take 24–48 hours, limiting acute management utility ### Supportive Investigations (Not Confirmatory) | Investigation | What It Shows | Limitation | |---|---|---| | Arterial blood gas + lactate | Metabolic acidosis, elevated lactate | Non-specific; seen in many poisonings | | Carboxyhaemoglobin | CO poisoning marker | Irrelevant to cyanide | | Urinary thiocyanate | Cyanide metabolite (appears after 3–5 days) | Too delayed for acute diagnosis | **Clinical Pearl:** While serum cyanide is the confirmatory test, clinical diagnosis must not be delayed waiting for results. Treatment with hydroxocobalamin or sodium thiosulfate should begin immediately based on clinical suspicion and supportive findings (metabolic acidosis, elevated lactate, normal oxygen saturation despite clinical hypoxia). **Mnemonic: CYANIDE CONFIRMATION** — **C**yanide level (serum), **Y**es to spectrophotometry, **A**rterial lactate (supportive only), **N**ormal O₂ sat (paradoxical), **I**mmediate treatment (don't wait), **D**elay in results (24–48 hrs), **E**arly clinical diagnosis. [cite:Parikh's Textbook of Medical Jurisprudence & Toxicology 7e Ch 18]
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