Aspartate Aminotransferase (AST) is an enzyme frequently included in routine blood tests, often as part of a liver panel or comprehensive metabolic panel. This measurement provides healthcare providers with insight into the health of various tissues. While a low level of AST is normal, an elevated result suggests that cells containing the enzyme have been damaged, causing its release into the bloodstream. Interpreting AST is key to understanding this common laboratory marker.
The Function and Location of AST
Aspartate Aminotransferase is a protein that plays a role in the metabolism of amino acids. Specifically, AST helps catalyze the reversible transfer of an amino group between aspartate and a keto acid, an important step in energy production. It is found inside cells, where it carries out its normal function, rather than working in the blood.
AST is highly concentrated in several organs, including the liver, skeletal muscles, the heart, and the kidneys. Since the enzyme is normally contained within these cells, its appearance in the bloodstream indicates that cellular integrity has been compromised. The level of AST detected often correlates with the extent of the cellular injury.
Interpreting AST Results
The concentration of AST is measured in units per liter (U/L). The normal reference range for healthy adults is typically between 8 and 40 U/L, though this can vary between laboratories. Factors like age, sex, and recent strenuous exercise can influence an individual’s baseline level. A result outside the established range is considered abnormal and warrants further investigation.
The degree of elevation provides an initial clue about the severity and potential cause of the underlying damage. Mildly elevated levels (less than five times the upper limit of normal) might be seen in chronic, less aggressive conditions or in response to intense physical activity.
Moderately elevated levels, reaching five to fifteen times the upper limit, typically point toward more progressive or acute forms of tissue injury.
Severely elevated AST levels, sometimes exceeding 15 times the normal upper limit, suggest a significant, acute cellular injury event. These very high numbers can indicate life-threatening conditions that have caused widespread cell death. The AST level is only one piece of the diagnostic puzzle and must be considered alongside a person’s symptoms and other blood markers.
Common Causes of Elevated AST
When AST levels are elevated, the cause is most frequently related to liver damage, given the organ’s high concentration of the enzyme. A range of liver conditions can lead to AST release, including acute and chronic inflammation (hepatitis). Viral infections (Hepatitis A, B, or C) or damage from certain prescription medications can cause hepatocellular injury and a subsequent rise in AST.
Chronic conditions such as alcohol-related liver disease and metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as non-alcoholic fatty liver disease (NAFLD), are common sources of persistently high AST. In later stages of liver disease, such as cirrhosis, extensive scarring and death of liver tissue lead to AST leakage. A sudden lack of blood flow to the liver, known as ischemic hepatitis or “shock liver,” can also cause a rapid and significant spike in AST levels.
Since AST is also present in non-liver tissues, its elevation is not solely diagnostic for liver issues. Significant trauma to skeletal muscles, such as rhabdomyolysis or extreme exercise, can release large amounts of AST into the blood. Similarly, a recent heart attack (myocardial infarction) damages cardiac muscle cells, which contain high levels of AST.
AST Versus ALT: Clinical Context
AST is almost always measured alongside Alanine Aminotransferase (ALT), another enzyme involved in amino acid metabolism. ALT is more specifically concentrated in the liver, making it a more precise indicator of liver damage compared to the more widely distributed AST. Analyzing both enzymes together provides a clearer clinical picture than looking at either value alone.
The ratio of AST to ALT, sometimes called the De Ritis ratio, helps differentiate the cause of the underlying liver injury. In many acute liver diseases, such as viral hepatitis or non-alcoholic fatty liver disease, the ALT level is typically higher than the AST level (ratio less than one). This pattern suggests a common form of liver inflammation.
A ratio of AST to ALT greater than 2:1 is highly suggestive of alcohol-related liver disease. This disproportionate elevation occurs because alcohol affects a specific type of AST found in the mitochondria of liver cells and is often linked to a vitamin B6 deficiency seen in heavy drinkers. A ratio greater than one can also be seen in advanced scarring of the liver (cirrhosis), regardless of the initial cause, or in cases of non-liver injury like muscle damage.