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Methemoglobin

Methemoglobin (MetHb, Hb M) is a structural hemoglobin (Hb) variant, which is formed when the heme portion of deoxygenated Hb is oxidized to a ferric state. When this occurs, the heme is incapable of combining with oxygen and transporting it to the tissues, and the patient becomes cyanotic.

Purpose

• To detect acquired methemoglobinemia, which is caused by excessive radiation or the toxic effects of chemicals or drugs
• To detect congenital methemoglobincmia.

Patient preparation

• Obtain a history of the patient's hematologic status and hemoglobin disorder, conditions that produce nitrites, plus exposure to sources of nitrites in drugs.
• Explain to the patient that this test is used to detect abnormal Hb in the blood.
• Tell him that a blood sample will be taken. Explain who will perform the venipuncture and when.
• Reassure him that drawing a blood sample will take less than 3 minutes.
• Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.
• Tell the patient to avoid medications that can interfere with test results.

Procedure and posttest care

• Perform a venipuncture, and collect sample in a 7-ml green-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• Completely fill the collection tube, and invert it gently several times.
• To avoid hemolysis, do not shake the tube vigorously.
• Place the collection tube on ice and send it to the laboratory immediately.

Reference values

Normal Met Hb levels are 0% to 1.5% of total Hb.

Abnormal findings

Increased MetHb levels may indicate acquired or hereditary methemoglobinemia, or carbon monoxide poisoning. These levels can also be caused by use of certain drugs or exposure to certain substances.

Decreased MetHb levels may occur in pancreatitis.

Interfering factors

• Acetanilid, aniline dyes, nitroglycerin, benzocaine, chi orates, lidocaine, nitrates, nitrites, phenacetin, sulfonamides, radiation, primaquine, and resorcinol (possible increase).
• Nitrite toxicity in breast-feeding infants as (increase due to conversion of inorganic nitrate to nitrite ion).

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