FIB-4 Index Calculator (Liver Fibrosis)
Estimate the likelihood of advanced liver fibrosis with the FIB-4 index — a simple, non-invasive score from age and three routine blood tests (AST, ALT and platelet count). It is widely used to triage people with fatty liver disease (MASLD/NAFLD) or hepatitis, helping to decide who can be reassured and who needs further assessment such as elastography. Enter the values in conventional units.
About the FIB-4 index
FIB-4 = (age × AST) ÷ (platelets × √ALT). It was developed in HIV/HCV co-infection and is now a first-line non-invasive fibrosis test in fatty liver disease and viral hepatitis. Source: Sterling RK et al., Hepatology 2006;43(6):1317–1325.
Frequently asked questions
What is the FIB-4 formula?
FIB-4 = (age in years × AST) ÷ (platelet count × √ALT), with AST and ALT in U/L and the platelet count in ×10⁹/L (the same as ×10³/µL). For example, a 60-year-old with AST 50, ALT 40 and platelets 150 has FIB-4 = (60 × 50) ÷ (150 × √40) ≈ 3.16. The platelet count sits in the denominator, so a falling platelet count pushes the index up just as a rising AST does. This calculator rounds the result to two decimal places and prints the matching risk band beneath it.
What do the FIB-4 results mean?
FIB-4 uses two cut-offs. Below 1.45 is low risk: advanced fibrosis is unlikely and the test has a high negative predictive value, so many people can be reassured. Above 3.25 is high risk: advanced fibrosis (stage F3–F4) is likely and referral for further assessment is warranted. Between 1.45 and 3.25 is indeterminate, usually prompting a second non-invasive test such as elastography.
Does age change the cut-offs?
Age does change how the lower cut-off should be read. In people over 65 the lower cut-off of 1.45 produces many false positives, so a higher lower threshold of about 2.0 is often recommended to keep the test specific. Below age 35 FIB-4 is less reliable. This calculator uses the standard 1.45 and 3.25 thresholds; interpret borderline results with age in mind.
What is FIB-4 used for?
FIB-4 is a first-line, non-invasive way to triage fibrosis risk in fatty liver disease (now called MASLD, previously NAFLD) and in chronic hepatitis B and C. Because it uses tests that are already routine, it is cheap and easy to repeat. A low value helps avoid unnecessary referrals; a high value flags people who need specialist assessment.
Can FIB-4 replace a liver biopsy?
FIB-4 cannot replace a liver biopsy. It estimates the probability of advanced fibrosis but does not stage it precisely, and it can be affected by anything that alters AST, ALT or platelets — acute hepatitis, alcohol, other cytopenias. It is a screening and triage tool used alongside elastography, imaging and clinical judgement, not a diagnosis. This calculator is an educational reference, not medical advice.
This is a reference tool, not a diagnosis or medical advice. Results depend on the laboratory, assay and clinical context, and reference intervals vary between labs and by age and sex — always read your result against the range printed on your own report and discuss it with a qualified healthcare professional.
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