The Aspartate Aminotransferase (AST) test is a common blood screening tool used to measure the level of this specific enzyme in the bloodstream. AST is also known by its older name, Serum Glutamic-Oxaloacetic Transaminase (SGOT). The test is frequently included as part of a routine comprehensive metabolic panel to assess general health and organ function. Elevated AST levels suggest that cells have been injured and released their internal contents into the circulation, providing insight into potential cell damage.
The Biological Role of AST
Aspartate Aminotransferase is an enzyme involved in the body’s metabolism, specifically helping to process amino acids. It catalyzes a chemical reaction that transfers an amino group, which is important for both the breakdown and creation of proteins. Although AST is often associated with liver health, it is widely distributed throughout the body in high concentrations.
The primary locations where AST is found include the liver, heart, and skeletal muscle, with smaller amounts present in the kidneys, brain, and red blood cells. In a healthy individual, only minimal amounts of this enzyme are detectable in the bloodstream. When cells in these organs are damaged due to injury or disease, their cellular membranes break down, allowing the AST enzyme to leak out. The resulting increase in circulating AST is what the blood test detects and measures.
Why Doctors Order the AST Test
The primary reason doctors order an AST test is to screen for and assess liver damage, as the liver contains high concentrations of the enzyme. It is often used in combination with other liver enzymes, most commonly Alanine Aminotransferase (ALT), to better evaluate liver health. This testing helps diagnose conditions such as viral hepatitis, cirrhosis, and metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as nonalcoholic fatty liver disease (NAFLD).
The test is also used for monitoring the progression of known liver conditions or evaluating treatment response. For individuals with risk factors like heavy alcohol use, diabetes, or a family history of liver disease, an AST test may be used as a screening measure. Although its role has been largely replaced by more specific markers like troponins, AST was historically used to help diagnose heart attacks because the enzyme is highly concentrated in cardiac muscle.
Interpreting Your Test Results
AST test results are reported in units per liter (U/L). The normal reference range typically falls between 8 and 33 U/L, though these numbers can vary slightly between laboratories. A result within the normal range suggests no significant acute damage to the organs that produce the enzyme. Elevated levels indicate that some degree of cell injury has occurred, and the magnitude of the elevation provides further clues.
A mild elevation (less than five times the upper limit of normal) is the most common finding and may be associated with conditions like fatty liver disease or certain medications. Extremely high AST levels (15 times or more above the normal range) usually point toward severe, acute events, such as an acetaminophen overdose, acute viral hepatitis, or significant shock to the liver.
The AST/ALT Ratio
The AST/ALT ratio compares the level of AST to Alanine Aminotransferase (ALT), an enzyme more specific to the liver. A ratio greater than 1 (AST higher than ALT) can suggest damage originating outside the liver, such as from muscle injury. However, a significantly elevated ratio of 2:1 or greater is a strong indicator of alcohol-related liver disease. Conversely, in many other forms of liver injury, such as acute viral hepatitis or MASLD, the ALT level is often higher than AST, resulting in a ratio less than 1.
Factors That Affect AST Levels
AST levels can be affected by various factors unrelated to chronic organ disease, potentially leading to temporary results. Strenuous physical activity, particularly intense weightlifting or endurance exercise, can cause temporary muscle breakdown, releasing AST into the bloodstream. This elevation is a consequence of muscle damage, not liver disease, and can skew test results for several days following the activity.
Certain medications are also known to cause transient increases in AST as a side effect, including some statins, nonsteroidal anti-inflammatory drugs (NSAIDs), and high doses of acetaminophen. Alcohol consumption, even light to moderate amounts, can influence enzyme levels, and heavy alcohol use is a direct cause of chronic elevation.