Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: Charles Patrick Davis, MD, PhD
Methemoglobinemia is a condition in which more than 1% of the hemoglobin in red blood cells takes the form of methemoglobin. Methemoglobulin is an abnormal form of hemoglobin (the molecule responsible for carrying oxygen in the blood cells) that is unable to bind to oxygen. A small amount of methemoglobin is normal (less than 1% of hemoglobin). Symptoms of anemia develop when this percentage increases over 10%, and levels of 70% are generally fatal.
Methemoglobin can be increased due to two main causes. Some cases of methemoglobinemia are genetic, meaning that an inborn metabolic error leads to an increased proportion of methemoglobin. Most cases of methemoglobinemia are, however, acquired rather than inborn. Exposure to certain oxidizing substances may lead to the conversion of hemoglobin to methemoglobin.