Aspartate Aminotransferase (AST)
A blood sample drawn from a vein in your arm
Aspartate aminotransferase (AST) is an enzyme found in cells throughout the body but mostly in the heart and liver and, to a lesser extent, in the kidneys and muscles. In healthy individuals, levels of AST in the blood are low. When liver or muscle cells are injured, they release AST into the blood. This makes AST a useful test for detecting or monitoring liver damage or related infections and some side effects of medications.
The liver is a vital organ located in the upper right-hand side of the abdominal area just beneath the rib cage. It is involved in many important functions in the body. The liver helps to process the body's nutrients, manufactures bile to help digest fats, produces many important proteins such as blood clotting factors, and breaks down potentially toxic substances into harmless ones that the body can use or excrete. It also helps metabolize some medications.
A number of conditions can cause injury to the liver and may cause increases in AST. The test is most useful in detecting liver damage due to hepatitis, drugs toxic to the liver, cirrhosis, or alcoholism. AST, however, is not specific for the liver and may be increased in conditions affecting other parts of the body.
An AST test is often performed along with an alanine aminotransferase (ALT) test. Both are enzymes found in the liver that become elevated in the blood when the liver is damaged. A calculated AST/ALT ratio is useful for differentiating between different causes of liver injury and in recognizing when the increased levels may be coming from another source, such as heart or muscle injury.
How is the test used?
The blood test for aspartate aminotransferase (AST) is usually used to detect liver damage and liver infections. It is often ordered in conjunction with another liver enzyme, alanine aminotransferase (ALT), or as part of a liver panel or comprehensive metabolic panel (CMP) to screen for and/or help diagnose liver disorders.
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 is often measured to monitor treatment of liver disease and may be ordered either by itself or along with other tests for this purpose.
Sometimes AST may be used to monitor people who are taking medications that are potentially toxic to the liver. If AST levels increase, then the person may be switched to another medication.
When is it ordered?
AST may be ordered as part of a comprehensive metabolic panel (CMP) when you have a routine health examination.
- Weakness, fatigue
- Loss of appetite
- Nausea, vomiting
- Abdominal swelling and/or pain
- Dark urine, light-colored stool
- Itching (pruritus)
- Swelling in the legs and ankles
- Tendency to bruise easily
AST may also be ordered, either by itself or with other tests, when you are at an increased risk for liver disease since many people with mild liver damage will have no signs or symptoms. You may be at increased risk if you:
- Have been exposed to hepatitis viruses
- Are a heavy drinker
- Have a history of liver disease in your family
- Are taking drugs that can damage the liver
- Are overweight and/or have diabetes
When AST is used to monitor treatment of persons with liver disease, it may be ordered on a regular basis during the course of treatment to determine whether the therapy is effective.
What does the test result mean?
Low levels of AST in the blood are expected and are normal.
Very high levels of AST (more than 10 times normal) are usually due to acute hepatitis, sometimes due to a viral 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 (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.
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.
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, hepatitis C virus-related chronic liver disease, 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.
What conditions other than liver problems can cause increased AST?
What other tests may be used to help determine the cause of liver damage?
After a thorough physical exam and evaluation of a person's medical history, there are several other tests that may be performed as follow up depending on what is suspected to be the cause of liver damage. Some of these include:
- Tests for hepatitis A, hepatitis B, and hepatitis C
- Testing for exposure to drugs and other substances toxic to the liver (see Drug Abuse Testing and Emergency and Overdose Drug Testing)
- Ethanol level
- Copper and ceruloplasmin for Wilson disease
- Iron tests and genetic tests for hereditary hemochromatosis
A liver biopsy may be performed to help determine the cause of liver injury and to evaluate the extent of liver damage.
For more information, see the article on Liver Disease.
Is there anything else I should know?
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 healthcare practitioner about all of the drugs and/or health supplements that you are taking.