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, 2, 3.
Required Laboratory Values
To calculate FIB-4, you need exactly four values 2:
- Age in years
- AST (aspartate aminotransferase) in IU/L or U/L
- ALT (alanine aminotransferase) in IU/L or U/L
- Platelet count in ×10⁹/L (or divide by 1000 if reported as ×10³/μL)
Step-by-Step Calculation
- Multiply the patient's age by their AST value 3
- Take the square root of the ALT value 1, 3
- Multiply the platelet count by the square root of ALT 3
- Divide the result from step 1 by the result from step 3 1, 2
Interpretation Thresholds
For most adult populations 2, 4:
- FIB-4 <1.3: Low probability of advanced fibrosis (negative predictive value ~90-95%) 2, 4
- FIB-4 1.3-2.67: Indeterminate range requiring additional testing with elastography or Enhanced Liver Fibrosis testing 2
- FIB-4 >2.67: High probability of advanced fibrosis (positive predictive value ~65%) 2, 4
Age-adjusted threshold for elderly patients 2:
- FIB-4 <2.0 for patients ≥65 years old to account for age-related increases in the score 2
Clinical Performance Characteristics
The FIB-4 index demonstrates an AUROC of 0.77-0.86 for detecting advanced fibrosis across different liver disease populations 2, 3, 4. The test performs best at ruling out advanced fibrosis rather than confirming it, with negative predictive values exceeding 90% 2, 4.
Important Caveats
Age significantly affects FIB-4 values, causing higher false-positive rates in elderly patients and potential false-negatives in younger patients 2. The score naturally increases with age independent of fibrosis progression 2.
In NAFLD populations specifically, diabetes and metabolic syndrome may reduce diagnostic accuracy 2. When using FIB-4 in cystic fibrosis-related liver disease, insufficient validation data exists for optimal thresholds 1.