Methemoglobinemia

Methemoglobinemia (or methaemoglobinaemia) is a disorder characterized by the presence of a higher than normal level of methemoglobin (metHb, i.e., ferric rather than ferrous haemoglobin) in the blood. Methemoglobin is an oxidized form of hemoglobin that has a decreased affinity for oxygen, resulting in an increased affinity of oxygen to other heme sites (that are still ferrous) within the same red blood cell. This leads to an overall reduced ability of the red blood cell to release oxygen to tissues, with the associated oxygen–hemoglobin dissociation curve therefore shifted to the left. When methemoglobin concentration is elevated in red blood cells, tissue hypoxia can occur.