The diagnosis of methemoglobinemia should be considered in all patients who present with cyanosis, and is particularly suspect in those whose cyanosis does not improve with supplemental oxygen. The blood of these patients has a characteristic "chocolate brown" color, analogous to that seen in the chocolate agar used to plate gonococcus (which contains methemoglobin). The ability to detect this discoloration is improved when compared directly to normal blood. 5 Pulse oximetry should be used cautiously in patients with methemoglobinemia. Because the pulse oximeter cannot properly differentiate oxyhemoglobin from methemoglobin, it may read an inappropriately normal value in patients with moderate methemoglobinemia, and it trends toward 85 percent in patients with severe methemoglobinemia. 6 Definitive identification of this abnormal hemoglobin species relies on co-oximetry, which is an in vitro spectrophotometric method that is capable of differentiating among oxy-, deoxy-, met- and carboxyhemoglobin species. This widely available test requires only a venous specimen, although arterial blood may be used if arterial puncture is indicated for another reason. The oxygen saturation obtained from a conventional arterial blood gas analyzer will also be falsely normal, because it is calculated from the dissolved oxygen tension, which is appropriately normal.
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