Methemoglobin (British English: Methaemoglobin) (pronounced "MET-hemoglobin") is a form of the oxygen-carrying proteinhemoglobin (British English: haemoglobin), in which the iron in the heme group is in the Fe3+ state, not the Fe2+ of normal hemoglobin. Methemoglobin is unable to carry oxygen. It is chocolate-brown in color. The NADH-dependent enzyme methemoglobin reductase(diaphorase I) is responsible for converting methemoglobin back to hemoglobin.
Normally one to two percent of people's hemoglobin is methemoglobin; a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems known as Methemoglobinemia. A higher level of methemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation.[1]
