Alanine Aminotransferase (ALT) vs. Aspartate Aminotransferase (AST)
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are two distinct enzymes—ALT is NOT the same as AST. 1
Key Differences Between ALT and AST
Tissue Distribution and Specificity
ALT is highly specific for liver injury because it is primarily concentrated in liver tissue with minimal presence in cardiac muscle, skeletal muscle, or red blood cells 1, 2
AST is significantly less liver-specific because it is present in cardiac muscle, skeletal muscle, kidneys, brain, and red blood cells, making it a less reliable marker for isolated hepatocellular injury 3, 1
ALT exists in low concentrations in skeletal muscle and kidney, whereas AST has substantial extrahepatic sources 1, 2
Clinical Utility and Interpretation
ALT is the preferred marker for determining liver disease severity and is used in most clinical guidelines for establishing treatment eligibility and monitoring 1
AST can be elevated in cardiac injury, skeletal muscle disorders (including exercise-induced elevation), hemolysis, and kidney disorders, requiring creatine kinase measurement to differentiate muscle from liver sources 3, 1
For mortality prediction, AST is paradoxically superior to ALT—elevated AST (≥40 IU/L) predicts 10.2 years of life expectancy loss compared to 5.2 years for elevated ALT, and shows stronger associations with all-cause mortality (HR=2.44 vs 1.69) and liver-related mortality (HR=27.2 vs 10.8) 4
Reference Ranges
Normal ALT ranges are sex-specific: 29-33 IU/L for males and 19-25 IU/L for females 1
These ranges are significantly lower than conventional commercial laboratory cutoffs 1
Clinical Decision-Making Based on AST/ALT Ratio
An AST/ALT ratio ≥2 is highly suggestive of alcoholic liver disease, with ratios >3 being particularly specific for this diagnosis 1
An AST/ALT ratio <1 is characteristic of nonalcoholic fatty liver disease (NAFLD), viral hepatitis, or medication-induced liver injury 1
In nonalcoholic disease, an AST/ALT ratio >1 strongly suggests cirrhosis and warrants evaluation for complications such as varices, ascites, and synthetic dysfunction 1
Practical Testing Recommendations
Use ALT as the primary screening marker for liver injury due to its superior hepatic specificity 3, 1
AST testing provides limited added clinical value when ALT is within reference intervals—73.2% of AST tests for males and 66.9% for females could be eliminated if AST were only performed when ALT is elevated 5
When both enzymes are elevated, check creatine kinase to exclude muscle injury as a source of transaminase elevation, particularly if the patient has engaged in intensive exercise 1