Clinical Tools · Hepatic Fibrosis · NASH / MASLD
FIB-4 Index Calculator
Calculate the FIB-4 liver fibrosis index from age, AST, ALT, and platelet count. Built for advanced fibrosis screening, NASH/MASLD trial context, and linkage to hepatic impairment assessment.
Quick Answer
The FIB-4 index estimates advanced liver fibrosis (Metavir F3–F4) from age, AST, ALT, and platelet count: FIB-4 = (Age × AST) / (Platelets × √ALT). Scores below 1.45 suggest low risk, 1.45–3.25 indeterminate, above 3.25 high risk. Pharma teams use FIB-4 for NASH/MASLD trial enrichment and hepatic impairment screening — a non-invasive alternative to biopsy when paired with elastography.
Calculate FIB-4 Index
Enter age, AST, ALT, and platelet count from the same clinical assessment window when possible.
Advanced fibrosis risk
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How to Use the FIB-4 Calculator
Worked Example
Inputs: age 52 years, AST 45 U/L, ALT 38 U/L, platelets 210 ×10⁹/L.
Calculation: FIB-4 = (52 × 45) / (210 × √38) = 2340 / (210 × 6.16) = 2340 / 1294 ≈ 1.81.
Interpretation: 1.81 falls in the indeterminate range (1.45–3.25). Consider transient elastography or specialist assessment per local pathway.
FIB-4 Interpretation Bands
Low likelihood of advanced fibrosis (F3-F4). Useful for ruling out advanced fibrosis in appropriate populations.
Intermediate probability. Further assessment with elastography, imaging, or biopsy may be warranted.
Higher likelihood of advanced fibrosis (F3-F4). Prompts specialist referral and additional staging in most pathways.
NASH / MASLD Context for Pharma Professionals
FIB-4 is widely used in metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD/NASH) screening algorithms and clinical trial operations. Sponsors may use FIB-4 thresholds to enrich populations (e.g., F2-F3 fibrosis by biopsy with FIB-4 corroboration), exclude patients with likely cirrhosis, or track non-invasive fibrosis trends alongside histologic endpoints.
FIB-4 estimates fibrosis risk; it does not replace Child-Pugh classification for hepatic impairment pharmacokinetics. When advanced fibrosis progresses to decompensated cirrhosis, drug development teams map hepatic impairment using Child-Pugh A/B/C in PK studies and labeling. Use our Child-Pugh Score Calculator for cirrhosis severity and dosing-context interpretation.
In registrational NASH trials, primary endpoints remain histology-driven (fibrosis stage improvement, NASH resolution). FIB-4 may appear as a secondary or exploratory biomarker, but regulatory acceptance as a surrogate endpoint requires dedicated validation programs.
Limitations and Caveats
FIB-4 performance varies by etiology, age, and comorbidity. Age in the numerator raises scores in older adults; platelets in the denominator amplify the effect of thrombocytopenia from portal hypertension but also confound when platelets are low for non-hepatic reasons.
Do not use FIB-4 during acute hepatitis flares, when AST/ALT reflect muscle injury, or when platelet count is unreliable. The indeterminate zone (1.45–3.25) has meaningful false-positive and false-negative rates—confirmatory testing is often required.
FIB-4 screens for advanced fibrosis risk; it does not stage mild fibrosis precisely, assess portal hypertension, or determine Child-Pugh class. Always integrate clinical context, imaging, elastography, and protocol-specific requirements.
Interpretation Reference Table
| FIB-4 score | Risk category | Advanced fibrosis (F3-F4) | Typical next step |
|---|---|---|---|
| < 1.45 | Low risk | Unlikely | Continue routine monitoring per guidelines; repeat if clinical context changes |
| 1.45 – 3.25 | Indeterminate | Uncertain | Consider FibroScan/elastography, specialist referral, or biopsy per pathway |
| > 3.25 | High risk | Likely | Hepatology referral, elastography, cirrhosis workup, variceal/HCC surveillance if confirmed |
Evidence & sources
- Sterling RK et al. Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV co-infection. Hepatology 2006.
- MDCalc: FIB-4 Index for Liver Fibrosis
- AASLD Practice Guidelines — MASLD/NAFLD management
- FDA guidance: pharmacokinetics in patients with impaired hepatic function (Child-Pugh context)
- Competitive landscape: MDCalc FIB-4 Index (#9 trending) provides validated cutoffs with hepatology evidence cards but emphasizes viral hepatitis biopsy deferral, not NASH/MASLD trial enrichment or Child-Pugh hepatic impairment dosing pathways. Merck Manual FIB-4 Calculator offers clinician-trusted reference scoring with 2026 MAFLD cutoffs but no pharma trial or registrational endpoint context. NovaPharmaNews adds NASH/MASLD sponsor workflow notes, FIB-4 vs APRI/FibroScan framing, and internal links to Child-Pugh for decompensated cirrhosis dosing decisions.