FIB-4 Liver Fibrosis Calculator
Estimate the risk of advanced liver fibrosis from four routine values, using the age-adjusted FIB-4 cutoffs.
Result
How it is calculated
The score is FIB-4 = (age × AST) ÷ (platelets × √ALT). Age is in years, AST and ALT in U/L, and platelets in ×109/L (the same number as ×1000 per µL, so 180 and not 180,000). The engine rounds to two decimals. It gives no result for an age under 18 or over 120, AST or ALT of zero or less, or platelets below 1 or above 10,000. The low cut-off is 1.30 under age 65 and 2.00 from age 65; the high cut-off is 2.67 for everyone. Below the low cut-off the label is low risk, between the two is indeterminate, above 2.67 is high risk, and above 3.25 a note about high specificity is added. These are screening bands for deciding on further assessment, not a diagnosis, and guidelines differ between conditions and countries.
Worked example
A 52-year-old has AST 48 U/L, ALT 40 U/L and platelets 180 ×109/L. Numerator: 52 × 48 = 2496. Denominator: 180 × √40 = 180 × 6.325 ≈ 1138.4. FIB-4 = 2496 ÷ 1138.4 = 2.19. The age is under 65, so the cut-offs are 1.30 and 2.67. Because 2.19 falls between them, the tool reports "Indeterminate: further assessment (elastography or ELF test) is advised." The same blood values at age 70 use the 2.00 low cut-off and give 2.95, which is high risk.
Common mistakes
- Entering platelets as 180,000 (cells per µL). The formula expects ×109/L, so the entry should be 180; a value that large gives no result.
- Swapping AST and ALT. AST goes in the numerator as it is, ALT is square-rooted in the denominator, so swapping them changes the score.
- Using the 1.30 cut-off for an older patient. From age 65 the tool switches the low cut-off to 2.00, so a score of 1.8 is indeterminate at 50 but low risk at 70.
- Entering the age in months or a child's age. The tool is for adults (18 to 120 years) and shows no result outside that range.
- Treating an indeterminate score as a negative test. It means more assessment is suggested.
About this calculator
The FIB-4 index is a non-invasive liver fibrosis screening score calculated from four routine values: FIB-4 = (Age × AST) / (Platelet count × √ALT). It was originally validated in hepatitis C/HIV co-infected patients but is now widely used across chronic liver disease, especially non-alcoholic fatty liver disease (NAFLD/NASH), to flag patients likely to have advanced fibrosis without needing a liver biopsy.
The standard cutoffs are age-adjusted because AST and platelet levels naturally shift with age: for patients under 65, a score below 1.30 suggests low risk of advanced fibrosis and above 2.67 suggests high risk, while patients 65 and older use a higher low-risk cutoff (below 2.0) since scores tend to run higher in that age group regardless of liver health; above 3.25 the specificity for advanced fibrosis is higher still — primary care doctors and hepatologists use FIB-4 as a first-line triage tool to decide who needs referral for further liver workup.
This calculator takes age, AST, ALT and platelet count and returns the FIB-4 score along with the age-adjusted fibrosis risk category.
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