How to Calculate the FIB-4 Score
Formula
The FIB-4 score is calculated using the formula: Age (years) × AST (IU/L) / [Platelet count (×10⁹/L) × √ALT (IU/L)] 1
Required Laboratory Values
To calculate FIB-4, you need exactly four values 1:
- Age in years
- AST (aspartate aminotransferase) in IU/L
- ALT (alanine aminotransferase) in IU/L
- Platelet count in ×10⁹/L (or multiply by 10⁹ if given in cells/μL)
Step-by-Step Calculation Example
- Multiply the patient's age by their AST level 1
- Take the square root of the ALT value 1
- Multiply the platelet count by the square root of ALT 1
- Divide the result from step 1 by the result from step 3 1
For instance, a 55-year-old patient with AST 45 IU/L, ALT 36 IU/L, and platelets 180 × 10⁹/L would have: (55 × 45) / (180 × √36) = 2475 / (180 × 6) = 2475 / 1080 = 2.29
Interpretation of Results
Once calculated, interpret the FIB-4 score using age-adjusted cutoffs 1, 2:
FIB-4 <1.3 (or <2.0 if age ≥65 years): Low probability of advanced fibrosis with approximately 90% negative predictive value—repeat testing in 2-3 years 1, 2
FIB-4 1.3-2.67: Indeterminate range requiring additional testing such as vibration-controlled transient elastography (FibroScan) or Enhanced Liver Fibrosis (ELF) testing 1, 2
FIB-4 >2.67: High probability of advanced fibrosis with approximately 60-80% positive predictive value—refer to hepatology for comprehensive evaluation 1, 2
Age-Specific Cutoff Refinements
For patients ≥60 years, age-adjusted cutoffs improve diagnostic accuracy 3:
- Ages ≤49 years: Low cutoff 1.05, high cutoff 1.21 3
- Ages 50-59 years: Low cutoff 1.24, high cutoff 1.96 3
- Ages 60-69 years: Low cutoff 1.88, high cutoff 2.67 3
- Ages ≥70 years: Low cutoff 1.95, high cutoff 2.67 3
These age-modified cutoffs reduce false-positive rates in elderly patients while maintaining sensitivity 3.
Clinical Performance Characteristics
The FIB-4 index demonstrates 4, 1:
- AUROC of 0.77-0.90 for detecting advanced fibrosis depending on the population studied 4
- Excellent negative predictive value (>90%) for ruling out advanced fibrosis 1
- Moderate positive predictive value (60-80%) for confirming advanced disease 2
Important Limitations and Pitfalls
Age significantly affects FIB-4 values, causing the score to naturally increase with age, leading to higher false-positive rates in elderly patients and potential false-negatives in younger patients 1. Always use the higher cutoff (<2.0) for patients ≥65 years to avoid overestimating fibrosis risk 2.
FIB-4 measures liver enzyme levels, which can generate false-positive results in patients with acute hepatitis independent of the degree of liver fibrosis 4. Avoid calculating FIB-4 during acute hepatic inflammation or flares of chronic liver disease.
The score performs poorly in patients younger than 35 years and may require adjusted cutoffs 2. Consider alternative testing modalities in very young patients.
Monitoring Fibrosis Progression Over Time
Serial FIB-4 measurements can track fibrosis progression 5. An increase in the ΔFIB-4 index per year ≥0.4 predicts progression to cirrhosis with an AUROC of 0.910, while ΔFIB-4/year <0.4 indicates stable disease 5. The cumulative incidence of progression to cirrhosis at 10 years is 59% with ΔFIB-4/year ≥0.4 versus only 3% with ΔFIB-4/year <0.4 5.