FIB-4 Liver Fibrosis Calculator

How it is calculated

FIB-4 combines four routine values into one index:

FIB-4 = (age × AST) / (platelets × √ALT)

Age is in years, AST and ALT are liver enzymes in U/L, and platelets are the count in ×10⁹/L (for example 220, not 220,000). The tool rounds the index to two decimals and applies two cut-offs: below 1.45 is shown as low risk of advanced fibrosis (F0–F1), 1.45 to 3.25 as indeterminate, and above 3.25 as high risk (F3–F4). All four values must be above zero, age must be 18–120 (it is an adult index), and a platelet figure above 5000 is rejected as a probable cells/µL slip, so no result appears. The same cut-offs apply at every age; some guidelines use different thresholds for older adults, and the score only guides follow-up testing such as elastography.

Common mistakes

• Entering the platelet count in cells per microlitre (220,000) instead of ×10⁹/L (220). Anything above 5000 is rejected and no result appears, so divide by 1,000 first.
• Swapping AST and ALT. AST goes in the numerator directly and ALT under a square root, so reversing them changes the score.
• Mixing lab units, such as AST and ALT in µkat/L from one lab. The formula expects U/L values.
• Reading "low risk" as proof there is no liver disease. The index only addresses advanced fibrosis (F3–F4), and normal enzymes do not exclude other problems.
• Treating the "indeterminate" band as a normal result. It means more testing is needed to decide.