FIB-4 Index (Liver Fibrosis)

About FIB-4 Index (Liver Fibrosis)

The FIB-4 index, introduced by Sterling and colleagues in Hepatology in 2006, is a non-invasive scoring system that estimates the probability of advanced liver fibrosis (stage F3-F4 on the METAVIR scale) using four routine laboratory values: age, AST, ALT, and platelet count. FIB-4 was originally developed for the assessment of liver fibrosis in patients with HIV-HCV co-infection, but it has since been validated in numerous other liver diseases, including non-alcoholic fatty liver disease (NAFLD), chronic hepatitis C, chronic hepatitis B, and alcoholic liver disease. The 2023 AASLD NAFLD guidance recommends FIB-4 as the first-line fibrosis assessment in primary care for all patients with suspected NAFLD, with referral to hepatology for FIB-4 ≥1.3 (or ≥2.67 in older adults).

What the calculator measures

FIB-4 estimates the probability that a patient has advanced fibrosis (METAVIR stage F3 or F4). The score is interpreted using two thresholds:

• <1.3: Low probability of advanced fibrosis (negative predictive value ~90%).
• 1.3-2.67: Indeterminate, requires further evaluation with elastography (FibroScan) or referral to hepatology.
• ≥2.67: High probability of advanced fibrosis (positive predictive value ~65-80%); requires hepatology referral.

In adults over age 65, the low-risk threshold is raised to <2.0 because FIB-4 naturally rises with age (due to declining platelets and rising AST), and the original threshold of 1.3 generates excessive false positives in older adults.

How the formula works

The FIB-4 formula is: FIB-4 = (age [years] × AST [U/L]) / (platelets [10⁹/L] × √ALT [U/L]). The formula reflects three biologic observations: (1) fibrosis progression is age-dependent; (2) AST tends to rise more than ALT in advanced fibrosis (AST/ALT ratio >1 is a marker of cirrhosis); and (3) platelet count declines as portal hypertension develops (due to splenic sequestration and reduced thrombopoietin production). The square root of ALT dampens the influence of ALT (which is more variable than AST) and prevents ALT from dominating the calculation. The formula was derived using multivariate analysis of clinical and laboratory variables in the HALT-C trial cohort (HIV/HCV co-infected patients). The four variables retained (age, AST, ALT, platelets) were the strongest independent predictors of advanced fibrosis in that cohort. FIB-4 has subsequently been validated in dozens of cohorts across multiple liver diseases.

Interpretation

FIB-4 interpretation depends on the patient population:

• Primary care NAFLD screening: FIB-4 <1.3 (or <2.0 in adults >65), low probability of advanced fibrosis; routine follow-up with lifestyle modification. FIB-4 ≥1.3 (or ≥2.0 in adults >65), further evaluation with elastography (FibroScan) or referral to hepatology.
• Hepatology referral: FIB-4 ≥2.67, high probability of advanced fibrosis; hepatology referral for further evaluation (elastography, MRE, or liver biopsy).
• HIV-HCV co-infection: FIB-4 ≥3.25, high probability of advanced fibrosis; consider direct-acting antiviral therapy regardless of other factors.
• Post-treatment HCV: FIB-4 should be re-assessed after HCV cure; some patients will have FIB-4 normalization, but those with established cirrhosis require ongoing hepatology surveillance (HCC screening, variceal screening).

Clinical use

FIB-4 is most useful in three scenarios. First, in primary care NAFLD screening, FIB-4 is the recommended first-line fibrosis assessment per the 2023 AASLD NAFLD guidance and the 2021 EASL-EASD-EASO NAFLD guideline. FIB-4 <1.3 in patients <65 (or <2.0 in patients >65) avoids unnecessary hepatology referral in the ~70-80% of NAFLD patients who do not have advanced fibrosis. FIB-4 ≥1.3 (or ≥2.0 in older adults) prompts further evaluation with elastography, which has higher accuracy but is not universally available in primary care. Second, in chronic hepatitis C, FIB-4 is used to assess fibrosis before and after direct-acting antiviral therapy. Pre-treatment FIB-4 ≥3.25 identifies patients with advanced fibrosis who require ongoing hepatology surveillance even after HCV cure. Post-treatment FIB-4 may improve, but patients with pre-treatment cirrhosis (F4) require ongoing HCC and variceal surveillance. Third, in chronic hepatitis B, FIB-4 is used to assess the need for antiviral therapy in patients with elevated ALT or HBV DNA. FIB-4 ≥2.67 supports treatment initiation even if ALT is normal.

Limitations

FIB-4 has several limitations. First, the index is influenced by non-hepatic conditions: thrombocytopenia from non-hepatic causes (e.g. ITP, chemotherapy, B12/folate deficiency), elevated AST from non-hepatic sources (e.g. muscle breakdown, hemolysis, thyroid disease), and elevated ALT from non-hepatic sources (e.g. muscle breakdown, thyroid disease). Second, FIB-4 has reduced accuracy in patients with normal or near-normal transaminases (the index was derived in patients with elevated transaminases). Third, FIB-4 has reduced accuracy in patients under age 35 (where the age numerator is small) and in patients over age 65 (where the age numerator is large and the original thresholds generate excessive false positives). The age-adjusted threshold of 2.0 for adults >65 partially addresses this. Fourth, FIB-4 is a probability estimate, not a definitive diagnosis; the gold standard for fibrosis staging is liver biopsy, but this is invasive and has sampling variability. Fifth, FIB-4 has lower accuracy than direct fibrosis biomarkers (e.g. Enhanced Liver Fibrosis panel) and elastography (FibroScan), which are preferred when available.

Comparison with other fibrosis scores

• FIB-4: Uses age, AST, ALT, platelets; widely available; recommended first-line in primary care.
• NAFLD Fibrosis Score (Angulo 2007): Uses age, BMI, hyperglycemia, AST/ALT ratio, platelets, albumin; similar accuracy to FIB-4 in NAFLD but more inputs.
• APRI (AST-to-Platelet Ratio Index): Uses AST and platelets; recommended by WHO for resource-limited settings.
• NFS (NAFLD Fibrosis Score): See above.
• Enhanced Liver Fibrosis (ELF) panel: Uses hyaluronic acid, PIIINP, TIMP-1; more accurate than FIB-4 but expensive and not universally available.
• FibroScan (transient elastography): Measures liver stiffness directly; more accurate than FIB-4 but requires specialized equipment.
• MRE (MR elastography): Most accurate non-invasive method; expensive and not universally available.
• Liver biopsy: Gold standard; invasive with sampling variability.

How to use the result

A FIB-4 of 1.8 in a 55-year-old with NAFLD is indeterminate and requires further evaluation with elastography (FibroScan). If the FibroScan shows liver stiffness <8 kPa, advanced fibrosis is excluded and the patient can be managed with lifestyle modification. If the FibroScan shows liver stiffness ≥10 kPa, advanced fibrosis is likely and hepatology referral is warranted for consideration of liver biopsy and treatment (e.g. resmetirom for MASH with F2-F3 fibrosis, approved by the FDA in 2024).