Methemoglobinemia

By Ilene B. Anderson, PharmD; and Susan Y. Kim, PharmD

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Laboratory Tests

The co-oximeter is the only reliable method to quantify oxygen saturation in a methemoglobin-poisoned patient. The co-oximeter uses spectrophotometric techniques to directly measure four different wavelengths of light corresponding to the absorbance of oxyhemoglobin, deoxyhemoglobin, methemoglobin, and carboxyhemoglobin. Co-oximetry measurement is available at most medical centers with intensive care capabilities.

Both the pulse oximeter and standard blood gas analysis will falsely report elevated oxygen saturation values in the methemoglobin-poisoned patient.¹⁵ The pulse oximeter relies on measurements at only two wavelengths of light typical for absorbance by oxyhemoglobin (660 nm) and deoxyhemoglobin (940 nm). Its computational algorithms assume that there are no other forms of hemoglobin present.^16,17 Methemoglobin absorbs more light at the 660-nm wavelength (oxyhemoglobin) than at the 940-nm wavelength (deoxyhemoglobin). Therefore, when the ratio of oxy/deoxyhemoglobin is calculated, an inaccurate result is reported. The reported oxygen saturation will be decreased, but reaches a plateau around 82 to 85% in the methemoglobin-poisoned patient.^17,18 The presence of methylene blue also leads to inaccuracies in pulse oximetry saturation estimates.

The standard blood gas machine uses the Pao₂ and the oxygen dissociation curve to calculate, rather than directly measure, the oxygen saturation. As with the pulse oximeter, the standard blood gas machine calculation is based on the presumption that no other forms of hemoglobin are present. Pao₂ reflects the total amount of oxygen present in blood, but it does not differentiate between oxygen bound to hemoglobin vs oxygen dissolved in blood yet not available for transport to the tissues; as a result, this measurement results in a falsely elevated value. Therefore, both of these methods, pulse oximetry and standard blood gas measurement, are unreliable methods for testing true oxygen saturation in the presence of Mhgb.

In contrast to methemoglobin, sulfhemoglobin is more challenging to quantify. Its measurement requires spectrophotometry or other laboratory methods that may not be readily available.

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