The clinical condition in which more than 1% of hemoglobin in blood has been oxidized to the ferric (Fe3+) form. The most common sign is cyanosis, because the oxidized hemoglobin does not transport oxygen. Very high concentrations of methemoglobin in the blood (i.e., greater than 30%) may produce dizziness, drowsiness, headache, or more severe neurological symptoms. Coma, seizures, and cardiac arrhythmias may occur with levels greater than 55%. Methylene blue is used as an antidote.
The existence of methemoglobin in the bloodstream.