Using Liver Enzymes AST and ALT to Diagnose Liver Damage and Disease

A Look at AST and ALT

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Liver enzymes are substances produced by the liver that can be measured through a blood test. Measurement of these enzymes circulating in your blood allows 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— aminotransferases AST and ALT—are especially useful for identifying liver damage and 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. 

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.

What Are Normal AST and ALT Levels?

The normal levels of AST and ALT vary from person to person and depend on your ​body mass index (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. People with acute viral hepatitis might have aminotransferase levels higher than 1,000 international units per liter of blood (IU/L).

Common normal laboratory ranges for AST and ALT are:

  • AST 2-45 IU/L
  • ALT 2-40 IU/L

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 help reach a diagnosis or order additional tests.

Abnormally high levels of both liver enzymes show that liver cells have been damaged, but they can't tell what caused the damage. 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.

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 what particular kind of liver disease may be present. 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.

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    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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