Methemoglobinemia: The presence in the blood of methemoglobin, a form of hemoglobin that is useless for carrying oxygen and delivering it to tissues throughout the human body. A small amount of methemoglobin is normally present in blood but the conversion of a larger fraction of hemoglobin into methemoglobin, which does not function well as an oxygen carrier, results in clinical symptoms. Since hemoglobin is the key carrier of oxygen in the blood, its replacement by methemoglobin can cause a slate gray-blueness of the skin (cyanosis), and potentially cause more serious symptoms due to insufficient oxygen.
Methemoglobinemia may be acquired anytime in life by exposure to a number of different chemical agents such as nitrites or certain medications (acquired methemoglobulinemia) or it may be present at birth (congenital) due a genetic condition.
REFERENCE: MedscapeReference.com. Methemoglobinemia in Emergency Medicine.