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Pulse oximetry in carbon monoxide poisoning — FRCA Primary MCQ

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HardPhysics and Clinical MeasurementPulse oximetry in carbon monoxide poisoningFRCA Primary

After smoke inhalation, pulse oximetry reads 99% but co-oximetry shows carboxyhaemoglobin 28%. What best explains the pulse-oximeter reading?

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Correct answer: C — Two-wavelength oximetry reads carboxyhaemoglobin similarly to oxyhaemoglobin

The best answer is “Two-wavelength oximetry reads carboxyhaemoglobin similarly to oxyhaemoglobin”. Standard two-wavelength pulse oximeters cannot reliably distinguish carboxyhaemoglobin from oxyhaemoglobin and may display a falsely reassuring saturation. PaO2 may be high on supplemental oxygen despite impaired oxygen carriage; co-oximetry is needed to quantify carboxyhaemoglobin.

Reference: RCoA, Primary FRCA Syllabus: https://rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf