Liver function tests (LFTs) are standard blood measurements used to assess liver health. Two routinely measured enzymes are Aspartate Aminotransferase (AST) and Alanine Aminotransferase (ALT). While elevated levels signal potential cell damage, the ratio of these two values provides a more specific diagnostic context. Calculating and interpreting the AST/ALT ratio helps clinicians narrow down the potential underlying cause of liver injury, guiding further investigation and treatment.
Understanding AST and ALT Enzymes
Both AST and ALT are aminotransferase enzymes involved in amino acid metabolism. They normally reside inside liver cells. When these cells are damaged or injured, the enzymes leak into the bloodstream, making their concentration a measure of hepatocellular damage.
Alanine Aminotransferase (ALT) is found predominantly in the liver, making it a more specific marker for liver injury than AST. Healthy adult reference ranges for ALT typically fall between 7 and 55 units per liter (U/L). Aspartate Aminotransferase (AST) is also abundant in the liver, but it is also found in other organs, including the heart, skeletal muscles, and kidneys.
The normal reference range for AST generally sits between 8 and 48 U/L. Elevated levels of both enzymes signal that liver cells have been compromised. While the degree of elevation suggests severity, the relationship between the two enzymes provides differentiating information about the cause.
The Simple Formula for Calculation
The AST/ALT ratio, sometimes called the De Ritis ratio, is calculated by dividing the measured concentration of Aspartate Aminotransferase by the measured concentration of Alanine Aminotransferase. The formula is: AST level / ALT level.
This ratio uses the raw numerical results from the blood test, measured in units per liter (U/L). For example, if a patient has an AST level of 80 U/L and an ALT level of 40 U/L, the ratio is 2.0. This resulting dimensionless number helps interpret the clinical picture.
The calculation’s simplicity allows for rapid assessment. However, the ratio’s diagnostic value relies on its pattern when considered alongside the patient’s medical history. It helps clinicians understand the likely type of damage, not just that damage has occurred.
Interpreting Different Ratio Values
The relative proportions of AST and ALT suggest the underlying cause of liver damage. A ratio of less than 1.0 (ALT higher than AST) is common in chronic liver injury, such as Non-Alcoholic Fatty Liver Disease (NAFLD) and chronic hepatitis C infection.
A ratio greater than 1.0 (AST exceeding ALT) suggests a more advanced stage of liver disease. This pattern is often seen when cirrhosis, or severe scarring of the liver tissue, has developed. The shift occurs due to changes in liver cell structure and the reduced clearance rate of AST as the disease progresses.
A significantly elevated ratio, specifically 2.0 or higher, is highly suggestive of Alcoholic Liver Disease (ALD). In ALD, the ratio is often at least 2:1, even if absolute enzyme levels are only moderately elevated. This occurs because alcohol-induced damage affects the mitochondria, where a large concentration of AST resides, leading to its disproportionate release.
When the ratio reaches 4.0 or greater, clinicians may consider highly specific conditions, such as Wilson’s disease, though this extreme elevation is rare. Interpretation is most reliable when enzyme levels are elevated, as a normal ratio with normal enzyme levels holds little diagnostic significance.
Non-Liver Factors Affecting Enzyme Levels
The AST/ALT ratio’s interpretation is complicated because both enzymes exist outside the liver. AST is abundant in skeletal muscle tissue, so muscle injury can raise its blood concentration. Intense physical activity, such as strenuous weightlifting or marathon running, can cause temporary, non-hepatic elevations in both AST and ALT, skewing the ratio.
Certain medications can affect liver enzyme levels, either by causing direct toxicity or altering enzyme metabolism. Examples include nonsteroidal anti-inflammatory drugs (NSAIDs) and statins. Additionally, the destruction of red blood cells (hemolysis) can elevate AST levels because the enzyme is present within those cells.
Other systemic conditions not directly related to the liver, such as Celiac disease or certain thyroid conditions, can also cause mild enzyme elevations. Due to these non-liver sources, a healthcare professional must evaluate the AST/ALT ratio within the context of the patient’s complete medical history, physical examination, and other laboratory results.