Methemoglobin is formed when iron in the hemoglobin molecule becomes oxidized from the ferrous to the ferric state. In this form, hemoglobin cannot bind oxygen. Methemoglobin levels are normally kept below 1.5% by an erythrocyte enzyme, methemoglobin reductase. Methemoglobinemia can be acquired or congenital. Acquired or toxic methemoglobinemia is much more common than congenital disease. It occurs following exposure to nitrites in food, nitrate in well water, or drugs containing an aniline ring such as sulfonamides, dapsone, Pyridium, local anesthetics, phenacetin, and acetaminophen. Household items containing aniline dyes include colored crayons, flavoring essences, furniture polish, ink, perfumes, and shoe polish. These agents oxidize ferrous hemoglobin at a rate that overwhelms the reductase enzyme. Signs and symptoms of methemoglobinemia may be delayed for several hours, because some chemicals do not directly oxidize hemoglobin but require hepatic metabolism to a toxic metabolite.

The most common congenital methemoglobinemia is deficiency of erythrocyte methemoglobin reductase. It is inherited as an autosomal recessive trait and occurs with increased frequency in Inuit and Alaskan Native Americans. Homozygotes have chronic cyanosis and methemoglobin levels of 15 to 20%, while heterozygotes are predisposed to toxic methemoglobinemia. Another cause of congenital methemoglobinemia is autosomal dominant inheritance of an abnormal M-hemoglobin. Chronic methemoglobinemia is not accompanied by erythrocytosis.

The severity of symptoms is proportional to the percentage of hemoglobin that has been oxidized to methemoglobin. Mild methemoglobinemia, with levels between 2 and 10% is well tolerated and is often asymptomatic in an otherwise healthy individual. Cyanosis, is the first sign of tissue hypoxia and occurs when methemoglobin levels rise above 15%. Cyanosis is characterized by a blue-gray appearance of the skin. Symptoms of more profound hypoxia begin to occur when methemoglobin levels exceed 20% and include headache, dyspnea, lightheadedness, weakness, confusion, palpitations and chest pain. Methemoglobin levels of 50 to 70% are associated with cardiac arrhythmias, altered mental status, seizures and metabolic acidosis. Levels above 70% may be fatal.

Arterial blood gas analysis is deceptive because the partial pressure of oxygen (PaO2) is normal in subjects with excessive levels of methemoglobin. A normal PaO2 in the presence of cyanosis is diagnostically useful. Administration of oxygen raises the PaO2 but fails to correct cyanosis in patients with methemoglobinemia. In contrast, oxygen therapy resolves cyanosis in patients whose cyanosis is secondary to cardiac or respiratory disease.

Oxygen saturation (sO2) reflects the oxygen bound to hemoglobin. Since methemoglobin cannot bind oxygen, methemoglobinemia is associated with reduced oxygen saturation. The oxygen saturation result that is reported with a blood gas analysis is falsely normal in patients with methemoglobinemia because it is based on a calculation that assumes a normal oxygen dissociation curve and the absence of dysfunctional hemoglobin.

Standard pulse oximeters measure tissue transmission at two wavelengths (660 and 940 nanometers) to calculate arterial sO2 as the ratio of oxyhemoglobin to total hemoglobin. The calculation assumes that only oxyhemoglobin is present. Oxygen saturation measured by pulse oximetry is decreased in patients with methemoglobinemia, but the reduction does not correlate with disease severity. Methemoglobin levels as high as 30% reduce sO2 from the normal level of 98% to approximately 85%. Higher levels of methemoglobin do not decrease sO2 any further.

The only reliable method of measuring methemoglobin is co-oximetry. A co-oximeter analyzes arterial blood at multiple wavelengths of light. Oxyhemoglobin exhibits very little optical density at a wavelength above 600 nm, while methemoglobin exhibits a small, characteristic peak at 630 nm.

A co-oximeter interprets all readings in the 630 nm range as methemoglobin. Sulhemoglobin often accompanies methemoglobin. The co-oximeter result may represent a combination of methemoglobin and sulfhemoglobin.

The presence of methemoglobin can be confirmed by the Evelyn Malloy test. The absorbance peak of methemoglobin is abolished with the addition of potassium cyanide, which converts methemoglobin to cyanmethemoglobin. The decrease in optical density is proportional to methemoglobin concentration.

Reference range is 0-1.5%.

Specimen requirement is one green top (lithium heparin) tube of blood or one blood gas syringe. The tube should be transported in wet ice. Methemoglobin is unstable and can degrade at a rate of about 40% per 24 hours.

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