Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are enzymes used to assess liver health. While the absolute levels of these enzymes indicate cell damage, the ratio between them provides context for diagnosis. This ratio offers insight into the potential cause and chronicity of liver injury, helping healthcare providers differentiate between various liver conditions.
The Role of AST and ALT Enzymes
AST and ALT are aminotransferases that help metabolize amino acids and generate energy within cells. They are typically contained within the cells of various organs. When cells are damaged, these enzymes leak into the bloodstream, where elevated levels can be detected. Alanine aminotransferase (ALT) is predominantly concentrated in liver cells (hepatocytes), making it a highly specific marker for liver injury. Aspartate aminotransferase (AST) is found in the liver but also in significant amounts in other tissues, including the heart, skeletal muscles, kidneys, and brain, making it a less specific indicator of liver damage than ALT.
Calculating the Ratio and Defining the Normal Range
The AST/ALT ratio, also known as the De Ritis ratio, is calculated by dividing the concentration of AST by the concentration of ALT. This comparative value helps clinicians characterize the pattern of injury. In a healthy individual, the normal AST/ALT ratio is typically less than 1.0, often falling between 0.7 and 1.2. This is because ALT has a longer half-life in the serum compared to AST, balancing the higher concentration of AST within the liver cell itself.
Diagnostic Significance of an Elevated Ratio
An elevated AST/ALT ratio occurs when AST concentration is significantly higher than ALT, often defined as greater than 1.0, and sometimes exceeding 2.0. A ratio of 2:1 or greater is strongly associated with alcoholic liver disease, including alcoholic hepatitis and cirrhosis. This pattern results partly from damage to the mitochondria within liver cells, which contain a large amount of AST. Furthermore, in alcoholic liver disease, a deficiency of the cofactor pyridoxine-5-phosphate can reduce ALT production, contributing to the disproportionate elevation of AST. An elevated ratio can also signal advanced liver cirrhosis from any cause or non-liver-related damage, such as severe muscle injury or a heart attack, due to AST’s presence in those tissues.
Diagnostic Significance of a Low Ratio
A low AST/ALT ratio (less than 1.0) indicates that ALT concentration is higher than AST, a pattern frequently observed in acute and chronic liver injury. This suggests hepatocellular disease where the injury is primarily contained within the cell cytoplasm. Since ALT is concentrated in the cytoplasm and is highly liver-specific, its disproportionate elevation signals acute or less severe liver inflammation. A low ratio is often associated with non-alcoholic fatty liver disease (NAFLD) and chronic viral hepatitis, such as Hepatitis C. In these conditions, the injury is less destructive to the mitochondrial components containing AST, meaning ALT leaks out more readily.