What FIB-4 measures
Chronic liver disease often progresses silently: scar tissue, or fibrosis, builds up for years before symptoms appear. The FIB-4 index uses four routine values, age, AST, ALT and platelet count, to estimate whether fibrosis is likely to be advanced. It costs nothing beyond a standard blood test and is now the recommended first step in most liver guidelines.
The formulas
- FIB-4 = (age × AST) ÷ (platelets × √ALT), with AST and ALT in U/L and platelets in 10³/µL (the same number as 10⁹/L).
- APRI = (AST ÷ upper limit of normal for AST) ÷ platelets × 100, an older index shown for comparison.
The logic: AST tends to rise relative to ALT and platelets fall as fibrosis advances, while age adds risk.
Worked example
A 55-year-old with AST 40 U/L, ALT 35 U/L and a platelet count of 200 × 10³/µL:
- √35 ≈ 5.92.
- FIB-4 = (55 × 40) ÷ (200 × 5.92) = 2,200 ÷ 1,183 ≈ 1.86.
- That is between 1.30 and 2.67: indeterminate, so a second test is advised.
- APRI = (40 ÷ 40) ÷ 200 × 100 = 0.50.
Reading the result
| FIB-4 | Meaning | Usual next step |
|---|---|---|
| Below 1.30 (below 2.0 from age 65) | Advanced fibrosis unlikely | Manage risk factors, repeat in 1 to 3 years |
| 1.30 to 2.67 | Indeterminate | Elastography (FibroScan) or an ELF blood test |
| Above 2.67 | Advanced fibrosis likely | Referral to a hepatologist |
The low cutoff has a negative predictive value of about 90% for advanced fibrosis in people with fatty liver disease, which is why it works well to rule out disease in primary care.
Who should be checked
The American Association for the Study of Liver Diseases (AASLD) recommends FIB-4 for adults with metabolic dysfunction-associated steatotic liver disease (MASLD), type 2 diabetes, obesity with metabolic risk factors, and those with two or more metabolic risk factors. It is also used in chronic hepatitis B and C and in alcohol-related liver disease.
Limits
- Not validated under age 35.
- Above 65, age pushes the score up; that is why the lower cutoff becomes 2.0.
- Acute hepatitis, recent heavy drinking, muscle injury (which raises AST) or a low platelet count from another cause can distort the result.
- FIB-4 estimates risk; it does not diagnose cirrhosis. Elastography or biopsy confirms the stage.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.