gastroenterology & hepatologyformula

FIB-4 Index

FIB-4 is a simple non-invasive index for liver fibrosis risk stratification calculated from age, AST, ALT, and platelet count. Formula: (Age × AST) / (Platelets × √ALT). It is widely used as a first-line triage tool for identifying patients who may have advanced hepatic fibrosis.

inputs

years
U/L
U/L
×10⁹/L

✓ when to use

Use as a first-line non-invasive fibrosis screening tool in patients with chronic liver disease, particularly NAFLD/MASLD, viral hepatitis (HCV, HBV), and alcoholic liver disease. Ideal for primary care triage — low FIB-4 can avoid unnecessary specialist referral, while high FIB-4 should prompt hepatology referral. Endorsed as step 1 in NICE and EASL fibrosis assessment pathways.

✗ when not to use

FIB-4 is less reliable in patients aged <35 (lower sensitivity — consider using a cut-off of <2.0 for low risk in younger patients) or >65 (reduced specificity — age inflates the score). Not validated in acute hepatitis (transaminases in flux), pregnancy, or conditions causing thrombocytopenia independent of liver disease (e.g., ITP, bone marrow disorders). FIB-4 assesses fibrosis, not disease activity or aetiology.

clinical pearls

  • FIB-4 is a triage test, not a diagnostic test. A low FIB-4 reliably excludes advanced fibrosis (high NPV), but a high FIB-4 needs confirmation with a second-line test (FibroScan, ELF) because its PPV is only ~65%.
  • The indeterminate zone (1.30–2.67) captures ~30% of patients. These are the ones who need second-line assessment. Do not simply repeat FIB-4 — escalate to elastography or specialist referral.
  • In patients >65, age alone inflates the FIB-4 numerator. Some guidelines suggest using a higher low-risk cutoff (e.g., <2.0) in older patients to reduce false positives, though this is not universally standardised.
  • FIB-4 is calculated from routine blood tests (FBC + LFTs), making it effectively free and available at every clinical encounter. This is its greatest advantage over FibroScan, which requires specialist equipment.
  • Monitor FIB-4 longitudinally in patients with NAFLD/MASLD — a rising FIB-4 over time may indicate progressive fibrosis even if the absolute value remains in the indeterminate zone.

related tools

nafld-fibrosisAlternative non-invasive fibrosis scorechild-pughCirrhosis severity stagingmeldLiver disease mortality prediction