What are the methods for testing cyanide toxicity?

Testing for Cyanide Toxicity

The most effective approach for diagnosing cyanide toxicity involves measuring blood lactate levels, with concentrations ≥8 mmol/L in isolated cyanide poisoning or ≥10 mmol/L in smoke inhalation victims being highly indicative of cyanide poisoning. 1, 2

Clinical Diagnosis

• Diagnosis of cyanide poisoning is often challenging as there is no widely available, rapid confirmatory blood test, requiring clinicians to rely on clinical history and presentation 1, 2
• Treatment decisions must be made based on signs, symptoms, and clinical suspicion rather than waiting for laboratory confirmation 3
• Common signs and symptoms that suggest cyanide toxicity include:
  • Neurological: altered mental status, confusion, disorientation, seizures, or coma 1
  • Cardiovascular: early hypertension followed by hypotension and cardiovascular collapse 1
  • Respiratory: early tachypnea/hyperpnea followed by bradypnea/apnea 1
  • Other: mydriasis, headache, dyspnea, chest tightness, nausea, and vomiting 1, 2

Laboratory Testing

• Elevated plasma lactate concentration (≥8 mmol/L) is a sensitive and specific indicator of cyanide poisoning 1, 2, 4
• In smoke inhalation victims, a lactate level ≥10 mmol/L is indicative of cyanide poisoning (higher threshold due to carbon monoxide's contribution to lactic acidosis) 1, 2
• Severe metabolic acidosis is common in serious cyanide poisoning cases 3, 4
• Gas chromatography/mass spectrometry can detect hydrazoic acid (formed when sodium azide contacts water) 3
• Infrared spectroscopy can be used to detect sodium azide 3
• Atomic absorption spectrometry with hydride generation can be used for related arsenic analysis 3

Special Considerations for Smoke Inhalation

• Not all smoke inhalation victims have cyanide poisoning; assessment should include: 1, 2
  • History of exposure to fire or smoke in an enclosed area
  • Presence of soot around mouth, nose, or oropharynx
  • Altered mental status
• Hypotension, though suggestive of cyanide poisoning, is present in only a small percentage of cyanide-poisoned smoke inhalation victims 1, 2

Important Caveats

• Do not delay treatment to obtain laboratory confirmation if clinical suspicion is high 3, 1, 2
• Headspace gas chromatography/mass spectrometry analysis can detect hydrazoic acid but may not be readily available in emergency settings 3
• Fixed dilated pupils should not preclude resuscitation efforts, as high success rates have been reported in such patients 3
• In cases of suspected cyanide poisoning from house fires, assess for concomitant carbon monoxide poisoning, which requires additional treatment 5

Treatment Considerations During Testing

• While diagnostic tests are being performed, immediate administration of 100% oxygen is recommended 3, 5
• For severe cardiotoxicity, treatment with hydroxocobalamin (Cyanokit) should be initiated without waiting for test confirmation 3, 5, 6
• Be aware that hydroxocobalamin can interfere with several clinical laboratory tests, potentially complicating interpretation of subsequent diagnostic results 6
• The combination of hydroxocobalamin followed by sodium thiosulfate is recommended for confirmed cases 3, 4

Remember that rapid identification and treatment are critical, as cyanide toxicity can progress quickly to respiratory arrest, cardiovascular collapse, and death within minutes 4, 7.