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AST/ALT Blood Test: What Is It For and Normal Levels

Updated in March 2022

AST and ALT, also known as transaminases, are two hepatic enzymes that are usually measured in the blood to assess liver health and functioning.

AST, also known as aspartate aminotransferase is stored in the hepatic cells of the liver, but is produced by other tissues like the heart and muscles. Therefore, when there is an increase only in the AST levels, it is usually related to other conditions not linked to the liver.

On the other hand, ALT, known as alanine aminotransferase, is produced exclusively in the liver and therefore when there is an increase in ALT levels, it normally indicates that there may be an alteration in the liver.

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Normal AST and ALT levels

AST and ALT values can vary according to the laboratory references, however, the levels that are considered normal are:

  • AST: 5 to 40 U/L;
  • ALT: 7 to 56 U/L.

Even though AST and ALT are considered liver markers, these enzymes can also be produced by other organs. Therefore, it’s important that the doctor who ordered the test, to confer these results with other lab values and a formal assessment to determine a diagnosis.

Possible causes for different blood levels

Alterations to both AST and ALT levels are normally indicative of liver injury, which can happen due to hepatitis, cirrhosis, or fatty liver.

On the other hand, when only AST levels are altered, it is possibly a sign of heart disease, since AST is also a cardiac marker. In these cases, the doctor may prescribe other tests to check cardiac health, such as troponin, myoglobin and creatine phosphokinase (CPK) blood tests.

In general, alterations to both AST and ALT levels may be related to the following situations:

  • Fulminant hepatitis;
  • Alcoholic hepatitis;
  • Cirrhosis due to excessive consumption of alcoholic drinks;
  • Abuse of illegal drugs;
  • Fatty liver;
  • Presence of a liver abscess;
  • Acute pancreatitis;
  • Bile duct obstruction;
  • Heart attack;
  • Heart failure;
  • Cardiac ischemia;
  • Muscle injury;
  • Use of medication for a long period and/or without medical supervision.

AST and ALT blood tests are normally ordered by doctors when one of the conditions listed above is suspected or when the patient has symptoms suggestive of a liver problem, such as yellow skin and eyes, dark urine, frequent fatigue or yellowish to white stool. Learn more about the symptoms that are usually associated with a liver problem.