How to Calculate the FIB-4 Score
The Formula
The FIB-4 score is calculated using the following formula: Age (years) × AST (U/L) / [Platelet count (×10⁹/L) × √ALT (U/L)] 1, 2.
Required Laboratory Values
To calculate the FIB-4 score, you need exactly four values 2:
- Age in years
- AST (aspartate aminotransferase) in U/L or IU/L
- ALT (alanine aminotransferase) in U/L or IU/L
- Platelet count in ×10⁹/L (or 10⁹/L)
Step-by-Step Calculation
- Multiply the patient's age by their AST level 1, 2
- Calculate the square root of the ALT value 1, 2
- Multiply the platelet count by the square root of ALT 1, 2
- Divide the result from step 1 by the result from step 3 1, 2
Interpretation Thresholds
For Patients ≤65 Years Old 1, 2:
- FIB-4 <1.3: Low probability of advanced fibrosis (negative predictive value ~90%) 2
- FIB-4 1.3-2.67: Indeterminate range requiring additional testing such as elastography 1, 2
- FIB-4 >2.67: High probability of advanced fibrosis (positive predictive value ~65%) 2
For Patients >65 Years Old 1, 2:
- The lower cut-off changes to 2.0 instead of 1.3, because FIB-4 naturally increases with age and leads to higher false-positive rates in elderly patients 1, 2
- FIB-4 <2.0: Low probability of advanced fibrosis in elderly patients 1
- FIB-4 ≥2.0: Requires further evaluation in elderly patients 1
Important Clinical Caveats
- Age significantly affects FIB-4 values, causing the score to naturally increase with age independent of fibrosis, which necessitates the different threshold for patients over 65 years 2
- The intermediate range (1.3-2.67) has limited accuracy, with approximately 30-40% of patients falling into this indeterminate zone where additional testing is mandatory 2
- FIB-4 performs poorly in younger patients (<35 years) and may produce false-negatives 2
- Diabetes and metabolic syndrome may reduce diagnostic accuracy in NAFLD populations 1, 2