Aspartate aminotransferase (AST) is a common enzyme marker measured in blood tests, frequently used to assess organ health. The concentration of this enzyme in the bloodstream is typically low in healthy individuals. Normal AST results vary naturally based on a person’s age, sex, and overall physical condition. Clarifying how these reference ranges shift throughout life is necessary for correctly interpreting a blood test result.
What is AST and Why is it Measured?
AST, or Aspartate Aminotransferase, is an enzyme previously known as Serum Glutamic-Oxaloacetic Transaminase (SGOT). Enzymes function as catalysts, speeding up chemical reactions within cells, and AST plays a role in amino acid metabolism. This enzyme is found in high concentrations in the liver, heart, skeletal muscles, and kidneys.
The measurement of AST in the blood is often performed as part of a liver function panel to detect cellular injury. When cells containing AST are damaged, their membranes break down, causing the enzyme to leak into the general circulation. Elevated levels of AST signal that cellular destruction or injury has occurred in one of the organs where the enzyme is concentrated.
Reference Ranges by Age Group
The definition of a normal AST level is not universal and changes significantly depending on the age of the individual. Reference intervals are established by laboratories to represent the values found in 95% of the healthy population, and these ranges reflect the different metabolic rates and growth phases across the lifespan. Results are typically reported in Units per Liter (U/L).
Newborns and infants often have the highest AST values, which can range from 35 to 140 U/L in the first few days of life. This temporary elevation is attributed to the rapid cellular turnover and metabolic adjustments occurring immediately after birth. These levels drop quickly as the infant ages.
During childhood and adolescence, the acceptable range begins to narrow but may remain slightly higher than in adults. Children between the ages of one and 13 may have upper limits around 50 U/L for girls and up to 60 U/L for boys. This difference is influenced by muscle mass, which increases more significantly in adolescent males.
For adults, the reference range typically falls to its lowest, often considered to be between 8 and 40 U/L. Adult males generally have a slightly higher upper limit than adult females due to their typically greater muscle mass. After the age of 60, the upper reference limits may increase slightly again.
Causes of Elevated AST Levels
An elevated AST result indicates cell damage, and the most common pathological causes are related to liver injury. Hepatitis, whether viral, alcoholic, or non-alcoholic fatty liver disease, causes inflammation that leads to the death of liver cells, or hepatocytes. Severe liver scarring, known as cirrhosis, also results in chronic inflammation and the continuous release of AST into the blood.
Extremely high AST elevations, sometimes exceeding one thousand U/L, may point to acute, severe damage, such as from an acetaminophen overdose or acute lack of blood flow to the liver, often termed “shock liver.” However, since AST is not exclusive to the liver, elevations can also signal damage to other organs. Damage to the heart muscle, such as during a myocardial infarction, or severe muscle injury, like rhabdomyolysis, releases large quantities of the enzyme. Other non-liver conditions, including acute pancreatitis, deep burns, and certain kidney diseases, can also lead to increased circulating AST.
Non-Disease Factors Affecting AST Results
Not every elevation in AST indicates a serious medical condition; many non-disease factors can cause temporary or mild increases. Intense physical activity, particularly strenuous weightlifting or endurance events, can cause minor skeletal muscle damage that transiently raises AST levels. The enzyme leaks from the strained muscle tissue, leading to a temporary increase that should resolve within a few days.
Certain medications are known to affect AST results, with some drugs causing temporary, mild elevations. These include common medicines like nonsteroidal anti-inflammatory drugs (NSAIDs) and cholesterol-lowering statins, which can sometimes impact liver cell function. Additionally, the process of drawing blood can cause a slight breakdown of red blood cells, known as hemolysis, which artificially inflates the AST reading since red blood cells contain the enzyme.