Aspartate Aminotransferase (AST)

Aspartate aminotransferase (AST) is an enzyme, also sometimes called aspartate transaminase (AST) or serum glutamic oxaloacetic transaminase (SGOT), that plays an important part in amino acid metabolism. In the human body, AST or AST is found in the liver, heart, kidneys, brain, skeletal muscles, and red blood cells. A test for serum AST is an important diagnostic tool for assessing possible liver disease.

Normal Levels

Normal levels of AST in the blood run between 6 and 40 IU per liter of blood. A blood test showing elevated levels of AST is therefore useful in identifying damage to the organs and tissues where AST is normally produced, which results in higher levels of the enzyme being released in the blood. Levels of AST in the blood as opposed to other tissues are normally low; when there is a significant increase it can indicate problems with the organs and tissues where AST is normally found as part of healthy function.

Aspartate aminotransferase is not as clear and unambiguous indicator of liver disease as elevated levels of alanine transaminase (ALT), because elevated AST can also indicate other diseases such as myocardial infarction, pancreatitis, hemolytic anemia, severe burns or other physical injuries, kidney disease, and diseases of the muscular or skeletal systems. However, the ratio of AST to ALT in cases where elevated levels of both are found can help identify the specific nature of the liver disease from which the patient suffers. The enzyme is also tested as part of an overall liver functioning test along with other enzymes.

Tests

A test for AST may be ordered if elevated ALT is found, or if a patient is suffering from any of several symptoms associated with liver disease, including weakness or fatigue, loss of apetite, nausea and vomiting, swelling or pain in the abdomen, jaundice (yellow coloring of the skin or the whites of the eyes), dark urine, light-colored stool, or itching (the latter without an obvious cause such as a rash or insect bites, of course). It may also be ordered because of known lifestyle or environmental factors, including alcohol abuse or alcoholism, exposure to viral hepatitis, family history of liver disease, obesity, or diabetes, all of these being risk factors for developing liver disease of one kind or another. The test may be ordered on a regular basis, along with others, to monitor the progress of treatment if liver disease is diagnosed.

Very High Levels Of AST

Very high levels of AST, more than ten times the expected maximum normal level, are usually associated with acute hepatitis (swelling of the liver). Acute hepatitis is most commonly caused by a viral infection. Levels of the enzyme can remain high for up to six months after symptoms of hepatitis and visual signs of the condition disappear.

Levels of AST elevation are generally lower with chronic liver diseases such as chronic hepatitis, blocked bile ducts, or cirrhosis of the liver and some liver cancers. AST can also be elevated after a heart attack or in the case of injury to the muscles, generally to a much greater degree than ALT, which is why comparing the two is useful in pinpointing liver disease after a test reveals elevated ALT, and also why an AST test by itself is not as useful for diagnosing liver disease as an ALT test –– it has many possible causes unrelated to the liver.

Use In Diagnosis

One important use of an AST test in diagnosis is after an elevated level of ALT is found, to help determine the type of liver disease likely present. The ratio of AST to ALT becomes important because, as noted above, levels of AST tend to be higher with some liver diseases than with others.

If the ratio of AST to ALT is more than double –– that is, the blood contains more than twice as much AST by volume as it does ALT –– this can indicate the presence of an acute liver disease such as viral hepatitis or alcoholic hepatitis, or liver cancer. When AST is elevated above ALT
but not as much as double, this is more likely to be a sign of a chronic liver condition such as cirrhosis. If AST is elevated to a lower level than ALT, this can show a form of hepatocellular disease.

Use In Follow-Up Testing

Because AST is a fairly reliable indicator of the degree of acute damage to the liver, follow-up testing for this enzyme can be useful in monitoring the progress of treatment for liver diseases. That’s especially true in monitoring treatment and recovery from acute liver diseases such as viral hepatitis; however, it’s also true in measuring the reduction in damage from lifestyle-related treatment of other liver diseases, in which the treatment attempts to remove the factors causing the problem by cessation of drinking, controlled weight loss, treatment of diabetes, changing of medications, or otherwise changing the lifestyle factors that contribute to the ongoing liver damage.