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Alanine Aminotransferase

This test is used to measure serum levels of alanine aminotransferase (ALT), one of two enzymes that catalyze a reversible amino group transfer reaction in the Krebs cycle. ALT is necessary for tissue energy production. ALT is found primarily in the liver, with lesser amounts in the kidneys, heart, and skeletal muscles, and is a sensitive indicator of acute hepato­cellular disease.

Purpose

• To detect and evaluate treatment of acute hepatic disease, especially hepatitis and cirrhosis without jaundice.
• To distinguish between myocardial and hepatic tissue damage (used with aspartate aminotransferase)
• To assess hepatotoxicity of some drugs

Patient preparation

• Explain to the patient that this test is used to assess liver function.
• Tell him that the test requires a blood sample. Explain who will perform the venipuncture and when.
• Explain that he may experience slight discomfort from the needle puncture and the tourniquet but that collecting the sample usually takes less than 3 minutes.
• Inform him that he needn't restrict food or fluids before the test.
• Withhold hepatotoxic and cholestatic drugs, such as methotrexate, chlorpromazine, salicylates, and narcotic analgesics. If these medications must be continued, note this on the laboratory slip.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml red-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Resume administration of medications that were withheld before the test.

Precautions

• Handle the sample gently to prevent hemolysis.
• ALT activity is stable in serum for up to 3 days at room temperature.

Reference values

Serum ALT levels range from 7 to 56 U/L.

Abnormal findings

Very high ALT levels (up to 50 times normal) suggest viral or severe drug. induced hepatitis or other hepatic disease with extensive necrosis. Moderate to high levels may indicate infectious mononucleosis, chronic hepatitis, intrahepatic cholestasis or cholecystitil, early or improving acute viral hepatitis, or severe hepatic congestion due to heart failure.

Slight to moderate elevations of ALT may appear in any condition that produces acute hepatocellular injury, such as active cirrhosis and drug­induced or alcoholic hepatitis. Marginal elevations occasionally occur in acute myocardial infarction, reflecting, secondary hepatic congestion or the release of small amounts of ALT from myocardial tissue.

Interfering factors

• Hemcmolysis due to rough handling of the sample.
• Barbiturates, griseofulvin, isoniazid, nitrofurantoin, methyldopa, phenothiazincs, phenytoin, salicylates, methotrexate, tetracycline, chlorpromazine, para-aminosalicylic acid, and other drugs that cause hepatic injury by competitively interfering with cellular mrlubolism (false-high).
• Nurcotic analgesics, such as morphine, codeine, and meperidine (possible false-high due to increased intrabiliary pressure)
• Ingestion of lead or exposure to carbon tetrachloride (sharp increase due to direct injury to hepatic cells).