Aspartate Aminotransferase Test

Definition
Purpose
Demographics
Description
Preparation
Aftercare
Risks
Normal results
Abnormal results
Morbidity and mortality rates
Alternatives Resources
Precautions
Side effects
Interactions

Definition

The aspartate aminotransferase test measures levels of AST, an enzyme released into the blood when certain organs or tissues, particularly the liver and heart, are injured. Aspartate aminotransferase (AST) is also known as serum glutamic oxaloacetic transaminase (SGOT).

Purpose

The determination of AST levels aids primarily in the diagnosis of liver disease. In the past, the AST test was used to diagnose heart attack (myocardial infarction or MI) but more accurate blood tests have largely replaced it for cardiac purposes.

Demographics

The number of AST tests administered each year can only be estimated. Since statins are the most prescribed drugs in the United States and standards of care call for quarterly liver function tests, the number of ASTs can easily exceed 500 million per year.

Description

AST is determined by analysis of a blood sample, usually taken from a venipuncture site at the bend of the elbow.

AST is found in the heart, liver, skeletal muscle, kidney, pancreas, spleen, lung, red blood cells, and brain tissue. When disease or injury affects these tissues, the cells are destroyed and AST is released into the bloodstream. The amount of AST is directly related to the number of cells affected by the disease or injury, but the level of elevation depends on the length of time that the blood is tested after the injury. Serum AST levels become elevated eight hours after cell injury, peak at 24–36 hours, and return to normal in three to seven days. If the cellular injury is chronic (ongoing), AST levels will remain elevated.

One of the most important uses for AST determination has formerly been in the diagnosis of a heart attack, or MI. AST can assist in determining the timing and extent of a recent MI, although it is less specific than creatine phosphokinase (CPK) , CK-MB, myoglobin, troponin, and lactic dehydrogenase (LDH). Assuming no further cardiac injury occurs, the AST level rises within 6–10 hours after an acute attack, peaks at 12–48 hours, and returns to normal in three to four days.

KEY TERMS

Cirrhosis— Disease of the liver caused by chronic damage to its cells.

Myocardial infarction— Commonly known as a heart attack. Sudden death of part of the heart muscle characterized, in most cases, by severe, unremitting chest pain.

Myocardial injuries such as angina (chest pain) or pericarditis (inflammation of the pericardium, the membrane around the heart) do not increase AST levels.

AST is also a valuable aid in the diagnosis of liver disease. Although not specific for liver disease, it can be used in combination with other enzymes to monitor the course of various liver disorders. Chronic, silent hepatitis (hepatitis C) is sometimes the cause of elevated AST. In alcoholic hepatitis, caused by excessive alcohol ingestion, AST values are moderately elevated; in acute viral hepatitis, AST levels can rise to over 20 times normal. Acute extrahepatic (outside the liver) obstruction, such as gallstones, produces AST levels that can quickly rise to 10 times normal, and then rapidly fall. In cases of cirrhosis, the AST level is related to the amount of active inflammation of the liver. Determination of AST also assists in early recognition of toxic hepatitis that results from exposure to drugs toxic to the liver, like acetaminophen and cholesterol-lowering medications.

Other disorders or diseases in which the AST determination can be valuable include acute pancreatitis, muscle disease, trauma, severe burn, and infectious mononucleosis.

Preparation

The physician may require discontinuation of any drugs that might affect the test. These types include such drugs as antihypertensives (for treatment of high blood pressure), coumarintype anticoagulants (blood-thinning drugs), digitalis, erythromycin (an antibiotic), oral contraceptives, and opiates, among others. The patient may also need to cut back on strenuous activities temporarily, because exercise can also elevate AST for a day or two.

Aftercare

This test involves blood being drawn, usually from a vein in the elbow. The person being tested should keep the wound from the needle puncture covered (with a bandage) until the bleeding stops. Individuals should report any unusual symptoms to their physician.

Risks

Risks for this test are minimal, but may include slight bleeding from the blood-drawing site, fainting or feeling lightheaded after venipuncture, or hematoma (blood accumulating under the puncture site).

Normal results

Normal ranges for the AST are laboratory-specific, but can range from 3–45 units/L (units per liter).

Abnormal results

Striking elevations of AST (400–4000 units/L) are found in almost all forms of acute hepatic necrosis, such as viral hepatitis and carbon tetrachloride poisoning. In alcoholics, even moderate doses of the analgesic acetaminophen have caused extreme elevations (1,960–29,700 units/L). Moderate rises of AST are seen in jaundice, cirrhosis, and metastatic carcinoma. Approximately 80% of patients with infectious mononucleosis show elevations in the range of 100-600 units/L.

Morbidity and mortality rates

Morbidity rates are excessively miniscule. The most common problems are minor bleeding and bruising. Since neither are reportable events, morbidity can only be estimated. Mortality is essentially zero.

Alternatives Resources

There are no alternatives to an aspartate amino-transferase test.

Precautions

The only precaution needed is to clean the venipuncture site with alcohol.

Side effects

The most common side effects of an AST test are minor bleeding and bruising.

Interactions

There are no known interactions with an AST test.

Resources

BOOKS

Fischbach, F. T. and M. B. Dunning. A Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams & Wilkins, 2008.

McGhee, M. A Guide to Laboratory Investigations, 5th ed. Oxford, UK: Radcliffe Publishing Ltd., 2008.

Price, C. P. Evidence-Based Laboratory Medicine: Principles, Practice, and Outcomes, 2nd ed. Washington, DC: AACC Press, 2007.

Scott, M. G., A. M. Gronowski, and C. S. Eby. Tietz’s Applied Laboratory Medicine, 2nd ed. New York: Wiley-Liss, 2007.

Springhouse Corp. Diagnostic Tests Made Incredibly Easy!, 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2008.

PERIODICALS

Inoue, K., M. Matsumoto, Y. Miyoshi, and Y. Kobayashi. “Elevated liver enzymes in women with a family history of diabetes.” Diabetes Research in Clinical Practice 79, no. 3 (February 2008): e4–e7.

Kansu, A. “Treatment of chronic hepatitis B in children.” Recent Patents on Anti-Infectious Drug Discoveries 3, no. 1 (January 2008): 64–69.

Lampe, E., C. F. Yoshida, R. V. De Oliveira, G. M. Lauer, and L. L. Lewis-Ximenez. “Molecular analysis and patterns of ALT and hepatitis C virus seroconversion in haemodialysis patients with acute hepatitis.” Nephrology (Carlton) 13, no. 3 (June 2008): 186–192.

Lazo, M., E. Selvin, and J. M. Clark. “Brief communication: clinical implications of short-term variability in liver function test results.” Annals of Internal Medicine 148, no. 5 (March 2008): 348–352.

OTHER

American Clinical Laboratory Association. Information about clinical chemistry. http://www.clinical-labs.org/ (February 24, 2008).

Clinical Laboratory Management Association. Information about clinical chemistry. http://www.clma.org/ (February 22, 2008).

Lab Tests Online. Information about lab tests. http://www.labtestsonline.org/ (February 24, 2008).

National Accreditation Agency for Clinical Laboratory Sciences. Information about laboratory tests. http://www.naacls.org/ (February 25, 2008).

ORGANIZATIONS

American Association for Clinical Chemistry, 1850 K Street, NW, Suite 625, Washington, DC, 20006, (800) 892-1400, http://www.aacc.org/AACC/.

American Society for Clinical Laboratory Science, 6701 Democracy Blvd., Suite 300, Bethesda, MD, 20817, (301) 657-2768, http://www.ascls.org/.

American Society for Clinical Pathology, 1225 New York Ave., NW, Suite 250, Washington, DC, 20005, (202) 347-4450, http://www.ascp.org/.

College of American Pathologists, 325 Waukegan Rd., Northfield, IL, 60093-2750, (800) 323-4040, http://www.cap.org/apps/cap.portal.

L. Fleming Fallon, Jr., M.D., Dr.P.H.