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Carbon Monoxide Poisoning — RACP Adult Medicine MCQ

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EasyEmergency MedicineCarbon Monoxide PoisoningRACP Adult Medicine

A 55-year-old man is found unconscious in a house fire. His carboxyhaemoglobin (COHb) level is 35%. SpO₂ by pulse oximetry reads 98%. What explains the discrepancy between SpO₂ and his clinical state?

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Correct answer: A — Pulse oximetry cannot distinguish COHb from oxyhaemoglobin, giving falsely normal readings

Standard pulse oximeters measure light absorption at two wavelengths and cannot distinguish carboxyhaemoglobin from oxyhaemoglobin. COHb absorbs light at a similar wavelength to oxyHb, causing falsely normal or elevated SpO₂ readings despite severe tissue hypoxia. Co-oximetry (ABG with co-oximetry) is required for accurate oxyhaemoglobin measurement. Treatment is 100% oxygen via non-rebreather mask (reduces COHb half-life from 4–5 hours to 60–90 minutes). Hyperbaric oxygen is considered for COHb >25% or neurological symptoms.

Reference: eTG – 2025 – Toxicology; ARC – 2024 – Carbon Monoxide Guidelines