FIB-4 Score Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The FIB-4 score is calculated with (age × AST) ÷ (platelets × √ALT). A score below 1.45 makes advanced liver fibrosis unlikely, while a score above 3.25 strongly suggests it.
Explanation
The FIB-4 score (Fibrosis-4 Index) is a non-invasive tool for assessing advanced liver fibrosis, complementary to our APRI score calculator: both scores use AST and platelets, but FIB-4 adds the patient's age and ALT (another liver enzyme), which generally improves its discriminative ability compared to APRI alone. The score rests on two well-established cutoffs: below 1.45, the negative predictive value is about 90% (sensitivity about 70%), making advanced fibrosis reasonably unlikely; above 3.25, specificity reaches about 97% (positive predictive value about 65%), strongly suggesting it. Between these two cutoffs lies a so-called indeterminate zone, where the score can't settle the question either way and a further test (elastography/FibroScan, or even liver biopsy) remains necessary. An important limitation to know: the FIB-4 score is less reliable in patients under 35, where its sensitivity drops (some guidelines suggest lowering the low cutoff to 2.0 in this group), and in patients over 65, where advanced age tends to artificially inflate the score and reduces its specificity — a result suggestive of fibrosis in an older patient therefore deserves cautious interpretation, accounting for this known limitation. Like APRI, this score remains a first-line screening tool, never a substitute for a healthcare professional's judgment or for imaging or biopsy when clinical suspicion persists — the same is true of our anion gap calculator, another first-line lab screening tool that likewise never replaces a full diagnosis.
Example: age 50, AST 80 U/L, platelets 150 G/L, ALT 40 U/L
Inputs
Age: 50. AST: 80 U/L. Platelets: 150 G/L. ALT: 40 U/L.
Calculation
FIB-4 = (50 × 80) ÷ (150 × √40) = 4,000 ÷ (150 × 6.3246) = 4,000 ÷ 948.68 ≈ 4.22.
Result
This FIB-4 score of 4.22 exceeds the 3.25 cutoff, strongly suggestive of advanced liver fibrosis.
Frequently asked questions
What is the difference between FIB-4 and the APRI score?
Both scores use AST and platelets, but FIB-4 adds the patient's age and ALT, which generally gives it a better ability to distinguish patients with and without advanced fibrosis. In clinical practice, both scores are often calculated together as complementary first-line screening tools, before considering a more thorough test if needed — see our APRI score calculator for that second tool.
Why is the FIB-4 score less reliable in young or elderly patients?
Because age sits directly in the numerator of the formula: in a young patient, a low score can underestimate real fibrosis (reduced sensitivity below 35), while in an older patient, the score can be artificially elevated without real advanced fibrosis (reduced specificity above 65), with age alone pushing the result upward. Some guidelines suggest adjusting the interpretation cutoffs by age group for this reason.
Does a score in the indeterminate zone (1.45-3.25) mean a liver problem?
Not necessarily: this zone simply means the FIB-4 score alone can't reliably settle the question either way. A further test (elastography/FibroScan, or liver biopsy in some cases) is then generally needed to clarify the situation, especially if other clinical or lab findings raise suspicion of liver disease.