Methemoglobin presence causes pulse oximetry to read approximately what saturation when true saturation is below 85%?

Methemoglobin presence causes pulse oximetry to read approximately what saturation when true saturation is below 85%?

• A. 100%
• B. 85%
• C. 50%
• D. 0%

Answer: (B)

Methemoglobinemia confounds pulse oximetry because the device estimates saturation by comparing light absorption at two wavelengths to distinguish oxyhemoglobin from deoxyhemoglobin. Methemoglobin (iron in the ferric state) absorbs light similarly at both wavelengths, so the algorithm converges on a value around 85% regardless of the true oxygenation when the actual saturation is low. As true saturation drops below about 85%, the SpO2 reading tends to plateau near 85% instead of decreasing further. This is why, in suspected methemoglobinemia, ABG with co-oximetry is needed to measure the actual saturation and MetHb level. The other values (100%, 50%, 0%) don’t reflect this typical plateau caused by methemoglobin.

Methemoglobinemia confounds pulse oximetry because the device estimates saturation by comparing light absorption at two wavelengths to distinguish oxyhemoglobin from deoxyhemoglobin. Methemoglobin (iron in the ferric state) absorbs light similarly at both wavelengths, so the algorithm converges on a value around 85% regardless of the true oxygenation when the actual saturation is low. As true saturation drops below about 85%, the SpO2 reading tends to plateau near 85% instead of decreasing further. This is why, in suspected methemoglobinemia, ABG with co-oximetry is needed to measure the actual saturation and MetHb level. The other values (100%, 50%, 0%) don’t reflect this typical plateau caused by methemoglobin.