Testing for AST is usually used to detect liver damage.

AST levels can be examined alongside other levels of liver enzymes, such as alkaline phosphatase (ALP), and alanine transaminase (ALT), to determine which form of liver disease is present.

Even though AST is found in heart and other muscles, another enzyme, creatine kinase (CK), is present in much higher amounts and is usually used to detect heart or muscle injury.

An AST test is requested with several other tests to help evaluate a patient who has symptoms of a liver disorder. Some of these symptoms include jaundice (yellowing of the eyes and skin), dark urine, nausea, vomiting, abdominal swelling, unusual weight gain, and abdominal pain. AST can also be ordered, either by itself or with other tests, for: 

• persons who might have been exposed to hepatitis viruses,
• those who drink too much alcohol,
• persons who have a history of liver disease in their family, or
• persons taking drugs that can occasionally damage the liver.

Persons who have mild symptoms, such as tiredness, may be tested for AST to make sure they do not have long-term (chronic) liver disease. AST is often measured to monitor treatment of persons with liver disease, and may be ordered either by itself or along with other tests.

Very high levels of AST (more than 10 times the highest normal level) are usually due to a rapidly developing liver disease called acute hepatitis, which is often due to a virus infection. In acute hepatitis, AST levels usually stay elevated for about 1–2 months, but can take as long as 3–6 months to return to normal. In the slowly developing variety of liver disease, chronic hepatitis, AST levels are usually not as high, often less than 4 times the highest normal level. In chronic hepatitis, AST often varies between normal and slightly increased, so doctors might request the test regularly to determine the pattern of change.

In some diseases of the liver, especially when the bile ducts are totally or partially blocked, or with cirrhosis and certain cancers of the liver, AST may be close to normal, but it increases more often than ALT. When liver damage is due to alcohol, AST often increases much more than ALT (this is a pattern seen with few other liver diseases). AST can be increased from break up of red blood cells (haemolysis), and is increased after heart attacks and with muscle injury.

Pregnancy may decrease AST levels. An injection of medicine into muscle tissue, or even strenuous exercise, may increase AST levels. In rare instances, some drugs can damage the liver or muscle, increasing AST levels. This is true of both prescription drugs and some 'natural' health products. If your doctor finds that you have high levels of AST, tell him or her about all the drugs and health products you are taking.