From the Guidelines
The Fibrosis-4 (FIB-4) index is calculated using the formula: age (yr) × AST (IU/L) / (platelet count (10^9/L) × √ALT (IU/L)) 1.
Calculation Components
- Age: in years
- AST: aspartate aminotransferase in IU/L
- Platelet count: in 10^9/L
- ALT: alanine aminotransferase in IU/L
Interpretation of FIB-4 Scores
- A FIB-4 score <1.45 has a negative predictive value of 90% for advanced fibrosis 1
- A FIB-4 score >3.25 has a positive predictive value of 65% for advanced fibrosis 1
- For individuals older than 65 years, a different lower FIB-4 cut-off of 2.0 applies 1
Clinical Application
The FIB-4 index is a useful, inexpensive, first-line assessment of liver fibrosis for use in primary care 1. It is recommended to re-assess FIB-4 every 1-3 years for individuals with a score <1.3 1.
Comparison with Other Biomarkers
FIB-4 outperforms other calculations, such as the aspartate aminotransferase-to-platelet ratio index, in its ability to identify patients with a low probability of advanced fibrosis 1. However, its ability to detect fibrosis is limited in the intermediate range (1.3-2.67), in the elderly, and in individuals with type 2 diabetes 1.
From the Research
Calculation of the Fibrosis-4 (FIB-4) Index
The FIB-4 index is a non-invasive tool used to assess liver fibrosis, calculated using the following formula:
- (age × aspartate aminotransferase [U/L]) / (platelet count [10^3/μL] × √alanine aminotransferase [U/L]) 2 This formula combines standard biochemical values (AST, ALT, and platelet count) and age to predict liver fibrosis.
Key Components of the FIB-4 Index
The FIB-4 index is based on the following key components:
- Age
- Aspartate aminotransferase (AST) levels
- Alanine aminotransferase (ALT) levels
- Platelet count These components are used to calculate the FIB-4 index, which can help identify patients with advanced fibrosis or cirrhosis 3, 4, 5.
Studies Supporting the FIB-4 Index Calculation
Several studies have validated the use of the FIB-4 index as a non-invasive tool for assessing liver fibrosis, including:
- A retrospective cross-sectional study of 128 NAFLD patients, which found that the FIB-4 index had 87% sensitivity and 87% specificity in detecting advanced fibrosis 3
- A study of 121 patients with NAFLD, which found that the FIB-4 index had an area under the receiver operating characteristic curve of 0.895 in detecting liver fibrosis 4
- A study of 847 liver biopsies from HCV-infected patients, which found that the FIB-4 index had an area under the receiver operating characteristic curve of 0.85 in detecting severe fibrosis 5