Knowledge check

Why is SpO2 (measured on pulse oximetry) inaccurate in methaemoglobinaemia?

Pulse oximetry compares light absorption at two wavelengths (660nm and 940nm; the wavelengths of maximum absorption by deoxygenated Hb and oxygenated Hb respectively) and uses the ratio to calculate the SaO2.

MetHb absorbs light equally at these two wavelengths and so distorts the ratio, trending it towards 1 which corresponds to a SpO2 of 85%.

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