Components of the FIB-4 Score
The FIB-4 score requires four readily available laboratory values: age (in years), AST (aspartate aminotransferase in IU/L), ALT (alanine aminotransferase in IU/L), and platelet count (in 10⁹/L). 1, 2
The FIB-4 Formula
The calculation is: Age (years) × AST (IU/L) / [Platelet count (×10⁹/L) × √ALT (IU/L)] 1, 2
Required Laboratory Components
- Age in years - This is a critical component that increases the numerator, making FIB-4 naturally rise with advancing age 2
- AST (aspartate aminotransferase) - Measured in IU/L or U/L 1
- ALT (alanine aminotransferase) - Measured in IU/L or U/L 1
- Platelet count - Measured in 10⁹/L (or 109/L) 1
Key Advantages of FIB-4 Components
FIB-4 is the most widely established and recommended first-line noninvasive test because all four components are simple, routinely available laboratory values that require no specialized testing or additional cost. 1, 3
The score can be calculated at bedside using free online calculators, making it immediately accessible in any clinical setting 1, 3
Special Considerations for HIV and MASLD Populations
FIB-4 was originally developed and validated in patients co-infected with HCV and HIV, making it particularly relevant for HIV populations. 1, 3
In HIV mono-infected patients, CD4+ cell count and albumin level are negatively associated with FIB-4 scores, while antiretroviral therapy (ART) is positively associated with higher FIB-4 scores 4
HIV RNA level has been associated with increased FIB-4 scores even in the absence of hepatitis B, hepatitis C, ART, or alcohol use, suggesting HIV infection itself may contribute to hepatic fibrosis 4
For MASLD patients with HIV, the same four components (age, AST, ALT, platelet count) are used with standard interpretation thresholds, though clinicians should be aware that HIV-related factors may influence the score. 1
Interpretation Thresholds
- FIB-4 <1.3 (or <2.0 if age ≥65 years): Low probability of advanced fibrosis 1, 3, 2
- FIB-4 1.3-2.67: Indeterminate range requiring additional testing such as elastography 1, 3, 2
- FIB-4 >2.67: High probability of advanced fibrosis warranting hepatology referral 1, 3, 2
Critical Pitfalls to Avoid
Always use the adjusted lower cutoff of 2.0 (instead of 1.3) for patients ≥65 years to avoid overestimating fibrosis risk due to age-related increases in the score. 1, 3
FIB-4 performs poorly in patients younger than 35 years due to age-dependent calculations 1
The score has limited accuracy in the intermediate range (1.3-2.67), where approximately 30-40% of patients fall, necessitating secondary testing with elastography or Enhanced Liver Fibrosis (ELF) testing 1, 2