FIB-4 Index Calculator
FIB-4 Index Calculator
Estimate the likelihood of advanced liver fibrosis from age, AST, ALT and platelet count — the first-line non-invasive test in fatty liver disease.
FIB-4 index
4 inputs → scoreAge 58, AST 62 U/L, ALT 45 U/L, platelets 180 ×10⁹/L
Formula
- Age
- years
- AST, ALT
- U/L
- Platelets
- ×10⁹/L (equivalent to ×10³/µL)
Worked example
Age 58, AST 62 U/L, ALT 45 U/L, platelets 180 ×10⁹/L
(58 × 62) ÷ (180 × √45) = 3596 ÷ (180 × 6.708) = 3596 ÷ 1207.5 = 2.98
Above 2.67 → advanced fibrosis likely
Interpretation thresholds
| Age band | Rule out | Indeterminate | Advanced fibrosis likely |
|---|---|---|---|
| 35 – 64 years | < 1.30 | 1.30 – 2.67 | > 2.67 |
| 65 years and over | < 2.00 | 2.00 – 2.67 | > 2.67 |
| Under 35 years | unreliable | unreliable | unreliable |
How to use the result
FIB-4 is a triage test, not a diagnosis. Its value lies almost entirely in its negative predictive value: a score below the lower cut-off makes advanced fibrosis unlikely enough that a patient can stay in primary care with periodic reassessment, which is exactly what a population-scale screening pathway for metabolic dysfunction-associated steatotic liver disease needs.
Scores in the indeterminate band — the largest group in practice — should proceed to a second non-invasive test, typically transient elastography or an ELF blood test, rather than to biopsy. Scores above 2.67 warrant hepatology referral.
Because age is a numerator term, FIB-4 drifts upward with age independently of any liver disease, and this is the reason for the modified cut-off above 65. In the other direction, anything that lowers the platelet count for a non-hepatic reason, or that raises AST from a muscle source, will inflate the score spuriously.
Common causes of a falsely elevated score are worth knowing before acting on one. Immune thrombocytopenia, hypersplenism from a non-hepatic cause, recent chemotherapy and marrow infiltration all depress platelets. Rhabdomyolysis, recent strenuous exercise, haemolysis and myocardial injury all raise AST without any hepatocyte involvement. In each of those situations the index is measuring something other than fibrosis, and the sensible response is to repeat the panel once the confounder has resolved rather than to escalate immediately.
Frequently asked questions
What is a normal FIB-4 score?
Below 1.30 in adults aged 35 to 64, and below 2.00 from 65 onwards. A score in that range makes advanced fibrosis unlikely, though it does not exclude steatosis or mild fibrosis.
Do I need to be fasting for a FIB-4?
No. FIB-4 uses a routine liver panel and full blood count, neither of which requires fasting for this purpose.
Why does FIB-4 use the square root of ALT?
The square-root term was chosen empirically during derivation to dampen the effect of very high ALT values, which reflect acute hepatocyte injury rather than accumulated fibrosis.
Can FIB-4 replace a liver biopsy?
Not on its own. It reliably identifies people who do not need further fibrosis work-up. Above the lower cut-off it is a reason to do a second, more specific test — elastography or ELF — not a reason to conclude the diagnosis.
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References
- Sterling RK, Lissen E, Clumeck N, et al. Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection. Hepatology. 2006;43(6):1317–25.
- McPherson S, Hardy T, Dufour JF, et al. Age as a confounding factor for the accurate non-invasive diagnosis of advanced NAFLD fibrosis. Am J Gastroenterol. 2017;112(5):740–51.
- AASLD Liver Fellow Network. Noninvasive assessment of patients with MASLD.