FIB-4 Score Calculation
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 the FIB-4 score, 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 per μL divided by 1000)
Step-by-Step Calculation Example
If you have a patient who is 55 years old with AST 60 IU/L, ALT 80 IU/L, and platelets 180 × 10⁹/L:
- Multiply age by AST: 55 × 60 = 3,300
- Calculate the square root of ALT: √80 = 8.94
- Multiply platelet count by √ALT: 180 × 8.94 = 1,609
- Divide the numerator by denominator: 3,300 ÷ 1,609 = 2.05
Interpretation of Results
Once calculated, interpret the FIB-4 score using these thresholds: 1
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 1-3 years 1, 2
FIB-4 1.3-2.67: Indeterminate range requiring additional testing such as liver elastography (FibroScan/VCTE) or Enhanced Liver Fibrosis (ELF) testing 1, 2
FIB-4 >2.67: High probability of advanced fibrosis with approximately 65% positive predictive value—refer to hepatology for comprehensive evaluation 1, 2
Age-Adjusted Cutoffs for Improved Accuracy
For patients ≥65 years, use a higher low-risk cutoff of <2.0 instead of <1.3 to avoid overestimating fibrosis risk due to age-related increases in the score. 1, 2, 3
Age-specific cutoffs that improve diagnostic accuracy in NAFLD include: 3
- Age ≤49 years: Low cutoff 1.05, high cutoff 1.21
- Age 50-59 years: Low cutoff 1.24, high cutoff 1.96
- Age 60-69 years: Low cutoff 1.88, high cutoff 2.67
- Age ≥70 years: Low cutoff 1.95, high cutoff 2.67
Important Clinical Caveats
The FIB-4 score naturally increases with age because age is in the numerator of the formula, leading to higher false-positive rates in elderly patients and potential false-negatives in younger patients. 1
FIB-4 performs poorly in patients younger than 35 years and requires adjusted cutoffs in those ≥65 years 2
The score has limited accuracy in the intermediate range (1.3-2.67), where approximately 30-40% of patients fall, necessitating additional testing 1
FIB-4 performs best at ruling out advanced fibrosis (high negative predictive value) rather than confirming it 1, 2
Serial Monitoring for Fibrosis Progression
Changes in FIB-4 over time (ΔFIB-4 index per year) can predict fibrosis progression, with an increase ≥0.4 per year associated with significantly higher risk of progression to cirrhosis. 4
- Patients with ΔFIB-4 index/year ≥0.4 had cumulative incidence of cirrhosis of 34% at 5 years and 59% at 10 years, compared to 0% and 3% in those with <0.4 per year 4