On pulse oximetry, carboxyhaemoglobin (COHb) results in SpO2 readings that are:

• A Falsely low, because COHb absorbs less light at 660 nm than oxyhaemoglobin
• B Accurate, because modern pulse oximeters are calibrated to exclude COHb
• C Variable depending on SpO2 level, with errors only occurring below 85%
• D Falsely high, because COHb is read as oxyhaemoglobin by the two-wavelength device ✓

Explanation

Standard pulse oximeters use only two wavelengths of light (660 nm red and 940 nm infrared). Carboxyhaemoglobin absorbs light very similarly to oxyhaemoglobin at 660 nm, so the device cannot distinguish between the two. As a result, COHb is counted as OxyHb, producing a falsely elevated SpO2 that does not reflect true oxygen saturation. This is clinically critical in carbon monoxide poisoning, where a normal-appearing SpO2 of 99% may coexist with a true SaO2 of 60–70%. Co-oximetry (measuring four or more wavelengths) is required to quantify COHb accurately.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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