Liver Enzymes: A Look at AST and ALT

From Charles Daniel, former About.com Guide
Updated September 05, 2008

About.com Health's Disease and Condition content is reviewed by the Medical Review Board

Liver enzymes allow doctors to learn about the health of your liver. There are thousands of these enzymes in the liver and blood stream, but two of them -- known as aminotransferases AST and ALT -- are especially useful for determining liver disease.

What Are Aminotransferases?

Aminotransferases are chemicals the liver uses to help make the energy-storage molecule glycogen. Aspartate aminotransferase, or AST, is found in the liver, but also the brain, pancreas, heart, skeletal muscle, kidneys and lungs. Alanine aminotransferase, or ALT, is primarily found in the liver. To remember the difference between the two, I use the "L" in ALT to remind me of the liver.

Why Are AST and ALT Useful?

Because these enzymes are found in liver cells (hepatocytes) which have lots of contact with your blood supply, AST and ALT can "leak" into the blood if the hepatocytes are damaged. Blood tests can determine the level of these enzymes in your blood and doctors can use this information to form a diagnosis. Abnormally high levels of both liver enzymes show that liver cells have been damaged, but they can't tell what caused the damage. Because AST is located in many places in the body, high levels of AST alone don't suggest liver disease (a notable, but rare, exception is Wilson's disease). However, the ratio of AST to ALT, or the level of AST compared to the level of ALT, provides many clues to what's going on inside. Based on these ratios, doctors can focus their attention on a particular kind of liver disease. Here are some common guidelines used for liver disease:

• An AST:ALT ratio equal to one (the level of ALT is higher or equal to AST), but the levels are very high, suggests acute viral hepatitis or drug-related hepatitis.
• An AST:ALT ratio higher than 2:1 (two times the level of AST to ALT) is very suggestive of alcoholic liver disease.
• An AST:ALT ratio higher than one (where the level of AST is higher than the ALT) could also indicate cirrhosis in a person that doesn't have alcoholic hepatitis.

Because increased enzyme levels can be seen in other diseases (heart attack, obesity, diabetes mellitus, mononucleosis), they are just one piece of a larger puzzle. To give doctors a complete clinical picture, enzyme levels must be used with other blood tests, patient examination and medical history.

• Diagnosing Hepatitis
• Blood Tests for Hepatitis C 

What Are the Normal AST and ALT Levels?

The normal levels of AST and ALT vary from person to person and depend on your BMI, or the ratio of your height and weight. A small amount of AST and ALT is normally in your blood, so it's really a large increase over your normal level that indicates a problem. For people with acute viral hepatitis, they might have aminotransferase levels higher than 1,000 IU/L. This stands for International Units per Liter (of blood) and is a unit of measurement that labs use for many medical tests. Common ranges for AST and ALT are:

• AST 2-45 IU/L
• ALT 2-40 IU/L

How Are Liver Enzymes Tested?

To test for AST and ALT, a doctor needs to order an enzyme test, which usually consists of several additional tests like albumin, bilirubin and prothrombin time. A technician will draw a sample of your blood and send it to a lab to be tested. The results are sent to your doctor.

Sources:
Pratt DS, Kaplan MM. Evaluation of Liver Function. In: AS Fauci, E Braunwald, DL Kasper, SL Hauser, DL Longo, JL Jameson, J Loscaizo (eds), Harrison’s Principles of Internal Medicine, 17e. New York, McGraw-Hill, 2008.

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