Apr 15, 2025
ALT vs AST: Key Differences Explained
Compare ALT and AST to learn which marker is more relevant for health assessment.
Introduction
ALT and AST are liver enzymes commonly used to evaluate liver function and injury. While they often rise together, they offer different clues depending on the tissue affected. This comparison explores their clinical differences.
What Is ALT?
ALT is a liver enzyme primarily found in the liver. It plays a key role in amino acid metabolism and is a sensitive marker for liver cell injury.
What Is AST?
AST is found in both the liver and other tissues like the heart and muscles. It helps assess liver damage but is less liver-specific than ALT.
Comparison Table
Criteria | ALT | AST |
|---|---|---|
Primary Function | Involved in amino acid metabolism; more specific to liver injury. | Also involved in amino acid metabolism; found in liver, heart, muscle. |
Diagnostic Relevance | Highly sensitive for detecting liver cell injury and inflammation. | Used in liver panels but less specific for liver damage alone. |
Speed of Response | Elevates quickly in response to liver injury or hepatitis. | Also elevates quickly, but may reflect injury from other tissues. |
Specificity | More liver-specific; elevated levels usually indicate liver disease. | Less specific; elevated in muscle injury, MI, or hemolysis as well. |
Clinical Use | Detecting hepatitis, liver inflammation, and fatty liver disease. | Used in conjunction with ALT to calculate the AST/ALT ratio. |
Measurement Units | U/L (units per liter) | U/L (units per liter) |
Normal Range | 7–56 U/L | 10–40 U/L |
Best Use Case | Monitoring liver-specific damage and disease. | Identifying systemic or non-liver causes of enzyme elevation. |
Clinical Use Cases
ALT is commonly used to detect early signs of liver injury, such as non-alcoholic fatty liver disease (NAFLD), viral hepatitis, or medication toxicity.
AST is often interpreted with ALT and is useful in evaluating broader organ damage, including cardiac or muscular injury, especially when ALT remains normal.
Which Test Should You Choose?
ALT is preferred for detecting liver-specific issues. AST is useful when you want a broader view, especially if muscle or heart damage is suspected. Both are typically ordered together for liver panels.
Using Both Together
The AST/ALT ratio is clinically important. A ratio above 2 often suggests alcoholic liver disease, while a low ratio favors NAFLD or viral hepatitis. Both markers provide more insight when analyzed together.
Pros and Limitations
ALT Pros: High liver specificity, reliable marker of hepatocellular injury.
ALT Limitations: Can miss non-hepatic sources of inflammation.
AST Pros: Reflects multiple organ systems and useful in multisystem evaluation.
AST Limitations: Lack of liver specificity limits diagnostic value in isolation.
When to Test
Both ALT and AST are tested when evaluating liver health, fatigue, jaundice, or during medication monitoring. They are also part of routine physicals and metabolic assessments.
FAQs
Why is ALT more specific than AST?
ALT is concentrated in the liver, while AST is found in multiple tissues.
What does a high AST/ALT ratio indicate?
It may point toward alcoholic liver disease or cirrhosis, especially if ALT is lower.
Can exercise affect AST and ALT?
Yes. Strenuous activity can elevate both, especially AST due to muscle involvement.