What is the FIB-4 formula?

FIB-4 = (Age [years] × AST [U/L]) ÷ (Platelet count [10⁹/L] × √ALT [U/L]). The result is a dimensionless number interpreted against standard thresholds.

What do the FIB-4 cutoff values mean?

A score below 1.30 suggests low fibrosis risk (F0–F1, NPV ~90%). A score above 2.67 suggests high fibrosis risk (F3–F4, PPV ~65%). Scores between 1.30 and 2.67 are indeterminate and typically require additional testing such as elastography or biopsy.

Is FIB-4 validated for all liver diseases?

FIB-4 was originally validated in HIV/HCV co-infection and chronic hepatitis C. It is also widely used in NAFLD/MASH and chronic hepatitis B, though cutoffs may differ slightly by population and guidelines.

What are the limitations of FIB-4?

FIB-4 can be elevated by non-hepatic causes of raised AST (e.g., muscle disease, hemolysis). It is less reliable in patients over 65, where the age term inflates the score. It does not replace clinical judgment or imaging.

Should I use FIB-4 alone to rule out advanced fibrosis?

No single test is definitive. FIB-4 is a first-line non-invasive tool. A low score can help reassure, but indeterminate or high scores should be confirmed with imaging (FibroScan/transient elastography) or liver biopsy.

What platelet unit does this calculator use?

Platelet count should be entered as 10⁹/L (also written as ×10⁹/L). This is the same as "thousands per microliter" (10³/μL) on most lab reports — for example, a result of 150,000/μL should be entered as 150.

FIB-4 Index Calculator

Name: FIB-4 Index Calculator
Availability: InStock
Author: Nham Vu

Enter age, AST, ALT, and platelet count to calculate the FIB-4 index and estimate liver fibrosis risk.

Patient Values

All values from a standard blood panel.

Age (years)

AST (U/L)

ALT (U/L)

Platelet Count (10⁹/L)

150,000/μL on your lab report = 150 here.

Interpretation Thresholds

Low risk (F0–F1) < 1.30

Indeterminate 1.30 – 2.67

High risk (F3–F4) > 2.67

Formula

FIB-4 = (Age × AST)
÷ (Platelets × √ALT)

Original reference: Sterling et al., Hepatology 2006.

Summary

Enter age, AST, ALT, and platelet count to calculate the FIB-4 index and estimate liver fibrosis risk.

How it works

Enter the patient's age in years.
Enter AST (Aspartate Aminotransferase) in U/L from a blood test.
Enter ALT (Alanine Aminotransferase) in U/L from a blood test.
Enter platelet count in 10⁹/L (× 10⁹ per liter).
The calculator applies the formula: FIB-4 = (Age × AST) / (Platelets × √ALT).
Your score is compared against validated thresholds to classify fibrosis risk.

Use cases

Screen for advanced liver fibrosis in patients with chronic hepatitis B or C.
Monitor fibrosis progression or regression in NAFLD/MASH patients.
Decide whether a liver biopsy is needed based on a non-invasive first-line score.
Triage patients in primary care for hepatology referral.
Track FIB-4 changes over time in response to treatment.
Use alongside APRI score for a second non-invasive fibrosis estimate.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu