FIB-4 Index Calculator (Liver Fibrosis)
The FIB-4 index is a simple, non-invasive scoring system that estimates the degree of liver fibrosis using four routine lab values: age, AST, ALT, and platelet count. It is recommended by the AASLD and EASL as a first-line screening tool and can often eliminate the need for liver biopsy.
The FIB-4 formula is: FIB-4 = (Age x AST) / (Platelet Count x √ALT)
Originally developed and validated in HIV/HCV co-infected patients, FIB-4 has since been validated across a wide range of chronic liver diseases including non-alcoholic fatty liver disease (NAFLD/MASH), chronic hepatitis B and C, and alcoholic liver disease. It is now one of the most widely used non-invasive fibrosis markers in clinical practice.
Interpreting FIB-4 scores:
- Below 1.30 (or below 2.0 for age 65+): Low risk of advanced fibrosis. The negative predictive value is approximately 90%, meaning 9 out of 10 patients in this range do not have significant fibrosis. No additional fibrosis workup is usually needed.
- 1.30 to 2.67 (indeterminate): Further testing is recommended. Transient elastography (FibroScan) is the most common next step.
- Above 2.67: High probability of advanced fibrosis (F3-F4). Hepatology referral and confirmatory testing should be pursued.
Why age-adjusted cutoffs matter: The standard low cutoff of 1.30 has reduced specificity in patients over 65 because age is in the numerator, pushing scores higher regardless of fibrosis status. Using a higher cutoff of 2.0 for older patients improves specificity without meaningfully reducing sensitivity.
Limitations: FIB-4 can be falsely elevated in acute hepatitis (transient AST spikes), hemolytic conditions (falsely low platelets or elevated AST), and after heavy exercise. It should be calculated using stable, non-acute lab values.
Disclaimer: This calculator is for educational purposes. Fibrosis assessment and management decisions should be made in consultation with a gastroenterologist or hepatologist.