How to Calculate the AST/ALT Ratio
The AST/ALT ratio is calculated by dividing the serum aspartate aminotransferase (AST) level by the serum alanine aminotransferase (ALT) level, using the same units (typically IU/L or U/L) for both values. 1
Calculation Method
- Simply divide AST by ALT: If AST = 152 U/L and ALT = 70 U/L, then AST/ALT ratio = 152 ÷ 70 = 2.17 1, 2
- Both values must be measured using the same laboratory method and expressed in the same units (IU/L or U/L) 3
- The calculation is straightforward arithmetic—no complex formulas or adjustments are needed 4, 1
Clinical Interpretation of the Ratio
The AST/ALT ratio provides critical diagnostic information about the underlying liver disease:
Alcoholic Liver Disease
- AST/ALT ratio ≥2.0 is strongly suggestive of alcoholic liver disease, with ratios >3 being particularly specific for this diagnosis 1, 2
- In alcoholic hepatitis, AST/ALT ratios >1.5 are seen in more than 98% of histologically proven cases 1
- The typical pattern shows AST around 152 U/L and ALT around 70 U/L, with mean ratio of 2.6 (range 1.1-11.2) 2
- AST and ALT typically do not exceed 400 IU/mL in alcoholic hepatitis 1
Nonalcoholic Fatty Liver Disease (NAFLD)
- AST/ALT ratio <1.0 is characteristic of NAFLD, viral hepatitis, or medication-induced liver injury 5, 2
- Patients with NASH have a mean AST/ALT ratio of 0.9 (range 0.3-2.8, median 0.7) 2, 6
- Mean AST levels are 66 U/L and mean ALT levels are 91 U/L in NASH patients 2
- An AST/ALT ratio <1 in all cases is highly suggestive of nonalcoholic steatohepatitis 6
Cirrhosis Detection
- AST/ALT ratio rising to >1.0 suggests the development of cirrhosis in patients with chronic nonalcoholic liver disease 7
- In chronic hepatitis B without cirrhosis, the mean AST/ALT ratio is 0.59, but rises to 1.02 when cirrhosis develops 7
- The ratio often rises to >1.0 when cirrhosis first becomes manifest 7
- In NASH patients, mean AST/ALT ratios are 0.7 for no fibrosis, 0.9 for mild fibrosis, and 1.4 for cirrhosis 2
Important Caveats and Pitfalls
Limitations of the Ratio
- The AST/ALT ratio becomes less reliable in the presence of cirrhosis, as the ratio may be >1.0 in nonalcoholic cirrhosis, potentially causing confusion with alcoholic liver disease 7
- AST is less liver-specific than ALT because it is present in cardiac muscle, skeletal muscle, kidneys, brain, and red blood cells 1, 5
- Recent intensive exercise or muscle injury can elevate AST more than ALT, artificially increasing the ratio 5
Special Considerations
- Normal ALT ranges differ by sex: 29-33 IU/L for males and 19-25 IU/L for females, which affects ratio interpretation 5
- In primary sclerosing cholangitis (PSC), an AST/ALT ratio ≥1 indicates cirrhosis with high specificity 1
- In Wilson disease with acute liver failure, AST may be higher than ALT, but this is accompanied by markedly low alkaline phosphatase 1
Methodological Considerations
- The JSCC consensus method correlates closely with both IFCC recommended method and Karmen method, with a slope of 1.00 and 0.87 respectively 3
- When using the JSCC consensus method, the decision-making value should be set to 0.87 instead of 1.0 used with the Karmen method 3
Practical Application Algorithm
When interpreting the AST/ALT ratio:
If ratio ≥2.0: Strongly suspect alcoholic liver disease; obtain detailed alcohol history (>50-60g/day for men, >40g/day for women for >6 months) 1, 8
If ratio <1.0: Consider NAFLD, viral hepatitis, or medication-induced liver injury; evaluate for metabolic syndrome components and review all medications 5, 2
If ratio >1.0 in nonalcoholic disease: Strongly suspect cirrhosis; evaluate for complications such as varices, ascites, and synthetic dysfunction 1
If ratio changes from <1.0 to >1.0 over time: This suggests progression to cirrhosis in chronic liver disease 7