Aspartate aminotransferase (AST) is a protein measured in the blood that acts as a biomarker for cellular injury. The AST blood test is frequently ordered as part of a comprehensive metabolic panel or a liver function panel. Measuring the level of this enzyme circulating in the bloodstream helps detect damage to certain organs. While it does not diagnose a specific disease, an elevated AST level indicates that cellular damage has occurred somewhere in the body.
The Role and Origin of Aspartate Aminotransferase
AST is an enzyme that plays a part in amino acid metabolism. It catalyzes the transfer of an amino group between aspartate and alpha-ketoglutarate, which helps the body process proteins. This function occurs entirely inside cells, not normally in the bloodstream.
The enzyme is found in high concentrations within the cells of several organs, most notably the liver, heart, skeletal muscle, and kidneys. Because AST is contained within these cells, it only leaks out into the circulation when those cells are injured or destroyed. The subsequent rise in the enzyme’s level in the blood serves as a signal of tissue damage.
Understanding AST Test Results
Results of an AST blood test are reported in units per liter (U/L) or international units per liter (IU/L). The normal reference range typically falls between 10 to 40 U/L for adults, though ranges vary by laboratory. An elevated AST level indicates that cell damage has occurred, causing the enzyme to spill into the blood.
A low AST result is far less common and not considered clinically significant on its own. Low levels may sometimes be observed in cases of chronic kidney disease or a deficiency in Vitamin B6, which is a cofactor the enzyme needs. When interpreting a high result, doctors often consider the AST level in relation to Alanine Aminotransferase (ALT).
While both enzymes reflect liver cell injury, ALT is primarily concentrated in the liver, making it a more specific indicator of liver damage. AST is less specific because of its presence in multiple organs. The ratio of AST to ALT (the AST/ALT ratio) can provide a diagnostic clue. For instance, a ratio greater than 2:1 is often associated with alcoholic liver disease, while a ratio less than 1 is more common in non-alcoholic fatty liver disease.
Primary Causes of Elevated AST Levels
The most frequent reason for an elevated AST level relates to injury within the liver, which contains the highest concentration of the enzyme. Various forms of hepatitis, including viral and autoimmune types, cause inflammation and destruction of liver cells, leading to enzyme release. Chronic conditions like cirrhosis or fatty liver disease (NAFLD) are also common causes of mild to moderate AST elevations.
Drug-induced liver injury is another cause, often resulting from certain prescription medications or over-the-counter pain relievers. An acetaminophen overdose, for example, can cause severe, acute liver cell death.
Elevated AST levels are not always caused by liver issues due to the enzyme’s widespread presence in other tissues. Significant damage to the heart muscle, such as during an acute myocardial infarction (heart attack), will cause a measurable spike in AST. However, this test has been largely replaced by troponin for cardiac assessment.
Severe trauma to skeletal muscle, a condition called rhabdomyolysis, can also cause a substantial elevation as muscle cells break down. Intense exercise can lead to a temporary rise in AST due to minor muscle fiber damage. Other non-liver causes include kidney damage, acute pancreatitis, and certain hematologic disorders. Because multiple organs can contribute to an elevated AST reading, further tests are necessary to pinpoint the exact source of the cellular damage.