AST & ALT Liver Enzyme Tests
🧬 AST & ALT Tests
The Most Sensitive Enzymatic Markers for Hepatocellular Injury — Complete Clinical Review
1. What Are AST and ALT?
✔ ALT (Alanine Aminotransferase)
- The most liver-specific aminotransferase
- Released into the bloodstream when hepatocytes are injured
- Located mainly in the cytosol (partially in mitochondria)
- A highly sensitive marker of hepatocellular damage
✔ AST (Aspartate Aminotransferase)
- Found in liver, heart, skeletal muscle, kidney, and RBCs
- Less specific for the liver
- Includes mitochondrial AST (mAST), which can surge in severe hepatic injury
- Elevation can also reflect muscle, cardiac, or hemolytic processes
2. Purpose of Testing
🔹 1) Evaluation of Hepatocellular Injury
- ALT: highly specific to hepatocyte damage
- AST: useful for determining liver vs. non-liver sources
🔹 2) Screening for Acute & Chronic Liver Diseases
Includes:
- Viral hepatitis (A, B, C)
- Alcoholic liver disease
- NAFLD / NASH
- Drug-induced liver injury (DILI)
- Toxin-related hepatotoxicity
🔹 3) Monitoring Liver Function Over Time
- Assessing treatment response
- Tracking drug-induced hepatotoxicity
- Baseline and post-treatment measurements
🔹 4) Evaluation of Alcoholic Liver Disease (AST/ALT ratio)
- AST/ALT ratio > 2–3:1 strongly suggests alcoholic injury
- Due to:
- Mitochondrial damage → elevated mAST
- Vitamin B6 deficiency → decreased ALT activity
3. Testing Method (Enzymatic Assay)
Most laboratories use automated UV kinetic enzymatic assays on high-throughput analyzers such as the Roche cobas 8000, which the author (you) routinely uses.
✔ ALT Measurement Principle
ALT catalyzes:
Alanine + α-ketoglutarate → Pyruvate + Glutamate
Then:
- LDH converts pyruvate → lactate
- NADH is oxidized to NAD⁺
- Rate of NADH decrease at 340 nm = ALT activity
✔ AST Measurement Principle
AST catalyzes:
Aspartate + α-ketoglutarate → Oxaloacetate + Glutamate
Then:
- MDH converts oxaloacetate → malate
- NADH is oxidized to NAD⁺
- NADH reduction rate at 340 nm = AST activity
✔ Advantages of enzymatic assay
- High accuracy & reproducibility
- Rapid reaction kinetics
- Full compatibility with automated instruments (e.g., cobas series)
4. Reference Ranges
Values differ among labs. Typical adult references:
ALT
- Men/Women: < 40 U/L
AST
- Men/Women: < 40 U/L
Each laboratory should follow its validated intervals.
5. Clinical Significance
🔺 ALT Elevation Causes
- Viral hepatitis (A, B, C)
- NAFLD / NASH
- DILI (antibiotics, NSAIDs, statins)
- Autoimmune hepatitis
- Wilson disease
- Ischemic hepatitis
- Early biliary obstruction
- Metabolic syndrome (obesity, diabetes)
🔺 AST Elevation Causes
- All liver diseases
- Alcoholic liver injury (AST/ALT > 2–3:1)
- Rhabdomyolysis, strenuous exercise
- Myocardial infarction, myocarditis
- Thyroid disorders
- Hemolysis
- Severe systemic illness (sepsis, renal failure)
🔻 Decreased AST/ALT (rare)
- Vitamin B6 deficiency
- End-stage burnt-out cirrhosis
- AST·ALT may appear “normal” despite advanced disease
6. AST/ALT Ratio (De Ritis Ratio)
Normal physiological ratio ≈ 1.15
✔ > 2 → Alcoholic liver disease
Due to:
- ↑ mAST release
- ↓ ALT from B6 deficiency
✔ < 1 → ALT-dominant pattern
Suggestive of:
- NAFLD / NASH
- DILI
- Viral hepatitis (early phase)
✔ > 1.15
- Severe necroinflammatory injury
- Acute hepatitis or ischemic hepatitis
7. Interpretation of Results
✔ ① ALT predominant elevation
Most common pattern → hepatocellular injury
- NAFLD/NASH
- DILI
- Early viral hepatitis
✔ ② AST predominant elevation
Consider non-hepatic causes:
- Muscle injury
- Cardiac disease
- Hemolysis
Confirm with CK, LDH, myoglobin.
✔ ③ AST ≫ ALT
- Alcoholic hepatitis
- Muscle injury
- Ischemic hepatitis / severe hepatic necrosis
✔ ④ AST or ALT > 1,000 U/L
Causes include:
- Acute viral hepatitis
- Severe DILI
- Ischemic hepatitis (“shock liver”)
- ALT >1,000 strongly suggests ischemic injury
8. Important Interpretation Caveats
1) DILI considerations
- Many antibiotics, statins, antiepileptics → ALT predominant increase
- Use R-value for cholestatic vs. hepatocellular pattern
2) Post-exercise AST elevation
- Common after strenuous exercise
- Check CK/LDH before assuming liver disease
3) Hemolyzed samples
- AST falsely elevated
- ALT relatively unaffected
- Ratio becomes misleading
4) Pregnancy
- Mild decrease early
- HELLP syndrome / acute fatty liver → sharp increases
5) Advanced cirrhosis
- AST/ALT may normalize due to hepatocyte depletion
- Must correlate with imaging and synthetic markers (INR, albumin)
9. Conclusion
AST and ALT remain the most widely used biochemical indicators of hepatocellular injury.
- ALT: liver-specific, sensitive
- AST: reflects liver + muscle + heart + RBC sources
The UV kinetic enzymatic assay is the standard method, especially on automated platforms such as the Roche cobas analyzers.
Accurate interpretation requires:
- AST/ALT ratio
- Pattern recognition (hepatocellular vs. muscle vs. alcohol use)
- Clinical context
- Additional tests when needed (CK, LDH, bilirubin, INR)
These enzymes are indispensable for evaluating NAFLD, alcoholic liver disease, DILI, acute hepatitis, and many systemic conditions.
📚 References
- Giannini EG, et al. ALT and AST in liver disease. Clin Liver Dis.
- Pratt DS, Liver chemistry and function tests. N Engl J Med.
- AASLD Guidelines – Abnormal Liver Tests
- WHO Laboratory Manual for Liver Function Testing
- Kim WR. AST/ALT ratio in clinical practice. Hepatology.
- Roche Diagnostics. ALT/AST Enzymatic Assay Methods (cobas platform)