For a patient with mild cyanosis, which abnormal hemoglobin is commonly found in elevated amounts?

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Mild cyanosis is often due to methemoglobinemia, where iron in the heme is in the ferric (Fe3+) state and cannot bind oxygen. Normally only trace amounts of methemoglobin are present and are kept in check by redox enzymes; when these systems are overwhelmed by oxidants or genetic defects, methemoglobin rises. This form of hemoglobin carries oxygen poorly and also reduces the amount of oxygen delivered to tissues, so tissue hypoxia can occur even with a normal arterial PO2. Clinically, this gives a blue-gray discoloration of skin and mucous membranes, and pulse oximetry may read around 85% regardless of true oxygenation. The other abnormalities are less commonly elevated in this setting: carboxyhemoglobin from CO poisoning produces a different color and presentation, sulfhemoglobin is a rarer cause of cyanosis, and Hemoglobin A is normal adult hemoglobin. Therefore, methemoglobin is the abnormal hemoglobin typically found elevated in mild cyanosis.