AST and ALT are considered to be two of the most important tests to detect liver injury, although ALT is more specific for the liver than is AST and is more commonly increased than is AST. Sometimes AST is compared directly to ALT and an AST/ALT ratio is calculated. This ratio may be used to distinguish between different causes of liver damage and to distinguish liver injury from damage to heart or muscle.
AST may also be ordered, either by itself or with other tests, for people who are at an increased risk for liver disease since many people with mild liver damage will have no signs or symptoms. Some examples include:
Very high levels of AST (more than 10 times normal) are usually due to acutehepatitis, sometimes due to a viral infection. With acute hepatitis, AST levels usually stay high for about 1-2 months but can take as long as 3-6 months to return to normal. Levels of AST may also be markedly elevated (often over 100 times normal) as a result of exposure to drugs or other substances that are toxic to the liver as well as in conditions that cause decreased blood flow (ischemia) to the liver.
With chronic hepatitis, AST levels are usually not as high, often less than 4 times normal, and are more likely to be normal than are ALT levels. AST often varies between normal and slightly increased with chronic hepatitis, so the test may be ordered frequently to determine the pattern. Such moderate increases may also be seen in other diseases of the liver, especially when the bile ducts are blocked, or with cirrhosis or certain cancers of the liver. AST may also increase after heart attacks and with muscle injury, usually to a much greater degree than ALT.
In most types of liver disease, the ALT level is higher than AST and the AST/ALT ratio will be low (less than 1). There are a few exceptions; the AST/ALT ratio is usually increased in alcoholic hepatitis, cirrhosis, and in the first day or two of acute hepatitis or injury from bile duct obstruction. With heart or muscle injury, AST is often much higher than ALT (often 3-5 times as high) and levels tend to stay higher than ALT for longer than with liver injury.
AST is often performed together with the ALT test or as part of a liver panel. For more about AST results in relation to other liver tests, see the Liver Panel article.
Pregnancy, a shot or injection of medicine into muscle tissue, or even strenuous exercise may increase AST levels. Acute burns, surgery, and seizures may raise AST levels as well.
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. Be sure to tell your health practitioner about all of the drugs and/or health supplements that you are taking.
This article was last reviewed on October 29, 2013. | This article was last modified on February 23, 2015.
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