Aspartate Aminotransferase

Test Description


Aspartate aminotransferase (AST) is an enzyme found primarily in the heart, liver,
and muscle. It is released into the circulation after injury or death of cells. AST levels
usually increase within 12 hours of the injury and remain elevated for 5 days.
Thus, this test is one of several that are performed when there has been damage to
the heart muscle, as in myocardial infarction, and in assessing liver damage. Other
cardiac enzymes also assessed are the creatine kinase (CK) isoenzymes, lactic
dehydrogenase (LDH, LD), and troponin.
AST is assessed along with alanine aminotransferase (ALT) in monitoring liver
damage. These two values normally exist in an approximately 1:1 ratio. The AST is
greater than the ALT in alcohol-induced hepatitis, cirrhosis, and metastatic cancer
of the liver. ALT is greater than AST in the case of viral or drug-induced hepatitis
and hepatic obstruction due to causes other than malignancy.
The degree of increase in these enzyme levels provides information as to the
possible source of the problem. A twofold increase is suggestive of an obstructive
problem, often requiring surgical intervention. A 10-fold increase of ALT and AST
indicates a probable medical problem such as hepatitis.

THE EVIDENCE FOR PRACTICE


In managing abnormal lipids, statin medications are commonly used. One major side effect
of statin use is liver toxicity, although the likelihood of liver transaminase elevations
>3 times the upper limit of normal is small. Liver transaminases (ALT and AST) are
obtained 6 to 12 weeks after statin therapy is initiated.


Normal Values

Female:                       9–25 U/L (0.15–0.42 μkat/L SI units)
Male:                         10–40 U/L (0.17–0.67 μkat/L SI units)
Elderly:                      Slightly higher norms
Newborn:                   Norms two to three times higher

Possible Meanings of Abnormal Values

Increased                                                       Decreased

Acute renal disease                                        Beriberi
Biliary obstruction                                         Diabetic ketoacidosis
Bone metastases                                            Hemodialysis
Brain trauma                                                  Pregnancy
Cancer of the prostate                                    Uremia
Cirrhosis
Eclampsia
Gangrene
Hemolytic disease
Hepatitis
Infectious mononucleosis
Liver cancer
Liver metastases
Liver necrosis
Malignant hyperthermia
Muscle inflammation
Myocardial infarction
Pancreatitis
Progressive muscular dystrophy
Pulmonary infarction
Reye’s syndrome
Shock
Severe burns
Trauma

Contributing Factors to Abnormal Values

• Drugs that may increase AST levels: acetaminophen, allopurinal, antibiotics, ascorbic
acid, chlorpropamide, cholestyramine, cholinergics, clofibrate, codeine, HMG-CoA
reductase inhibitors, hydralazine, isoniazid, meperidine, methyldopa, morphine, oral
contraceptives, phenothiazines, procainamide, pyridoxine, salicylates, sulfonamides,
verapamil, vitamin A.
• Drugs that may decrease AST levels: metronidazole, trifluoperazine.

Interventions/Implications

Pretest

• Explain to the patient the purpose of the test and the need for a blood sample to be drawn.
• When assessing for myocardial infarction, this test is often performed on 3 consecutive
days, and again in 1 week, necessitating multiple venipunctures.
• No fasting is required before the test.

Procedure

• A 7-mL blood sample is drawn in a red-top collection tube.
• Gloves are worn throughout the procedure.

Posttest
• Apply pressure 3–5 minutes at venipuncture site. Apply dressing, periodically assessing
for continued bleeding.
• Teach the patient to monitor the site. If the site begins to bleed, the patient should apply
direct pressure and, if unable to control the bleeding, return to the laboratory.
• Label the specimen and transport it to the laboratory.
• Report abnormal findings to the primary care providers.

Clinical alert                 
• Patients with liver dysfunction may have prolonged clotting time.
• Liver enzymes, including ALT and AST, are routinely monitored in patients who
  take HMG-CoEnzyme A reductase inhibitors (“statin” medications).