AST and ALT are considered to be two of the most important tests to detect liver injury, although ALT is more specific than AST. AST compared directly to ALT yields an AST/ALT ratio, which distinguishes between different causes of liver damage.
AST helps to detect liver damage. AST levels are compared with results of other tests, such as ALP, total protein and bilirubin to help determine which form of liver disease is present. AST assists in monitoring treatment of patients with liver disease. AST helps monitor people who are taking medications that are potentially toxic to the liver. If AST levels increase, then a change to a different medication will be considered.
Normally, levels of AST in the blood are low. Very high levels of AST (more than 10 times the highest normal level) are usually due to acute hepatitis, often due to a virus infection. In 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 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.
AST levels are usually not as high in chronic hepatitis, often less than 4 times the highest normal level. In chronic hepatitis, AST often varies between normal and slightly increased, so doctors typically will order the test frequently to determine the pattern. Such moderate increases may indicate certain other diseases of the liver, especially when the bile ducts are blocked, or with cirrhosis and certain cancers of the liver. AST may also increase after heart attacks and with muscle injury, usually to a much greater degree than ALT.
ALT helps detect liver injury. ALT values, compared to the results of other tests such as alkaline phosphatase (ALP), total protein and bilirubin, to help determine which form of liver disease is present. ALT levels help monitor the treatment of patients who have liver disease, to see if the treatment is working. Drugs that may increase ALT levels include acetaminophen, ampicillin, codeine, dicumarol, indomethacin, methotrexate, oral contraceptives, tetracycline and verapamil. Previous intramuscular injections may cause elevated levels.
The AST/ALT ratio is sometimes useful in differentiating between causes of liver damage (hepatotoxicity). The ratio is less useful in scenarios where the liver enzymes are not elevated, or where multiple conditions co-exist.In most types of liver disease, the ALT level is higher than AST and the AST/ALT ratio is a value less than 1.0. There are a few exceptions. The AST/ALT ratio greater than 1.0 in alcoholic hepatitis, cirrhosis and in the first day or two of acute hepatitis or injury from bile duct obstruction. A value greater that 2.0 indicates alcoholic liver disease.