How do I calculate the FIB‑4 (Fibrosis‑4) score using a patient's age, AST, ALT, and platelet count?

How to Calculate the FIB-4 Score

The FIB-4 score is calculated using the formula: Age (years) × AST (U/L) / [Platelet count (×10⁹/L) × √ALT (U/L)] 1.

Required Laboratory Values

You need four routine laboratory parameters:

• Age in years
• AST (aspartate aminotransferase) in U/L
• ALT (alanine aminotransferase) in U/L
• Platelet count in ×10⁹/L (or ×10³/μL)

Step-by-Step Calculation

1. Multiply the patient's age by their AST level
2. Calculate the square root of the ALT value
3. Multiply the platelet count by the square root of ALT
4. Divide the result from step 1 by the result from step 3

Clinical Interpretation

The FIB-4 index is used to assess liver fibrosis severity across multiple chronic liver diseases including hepatitis B, hepatitis C, and metabolic dysfunction-associated steatotic liver disease (MASLD) 2, 3.

For ruling out advanced fibrosis:

• FIB-4 <1.45 has a negative predictive value of 90-95% to exclude advanced fibrosis (stage F3-F4), with sensitivity of 70-73% 3, 1, 4
• This cutoff can reliably exclude advanced fibrosis and avoid liver biopsy in approximately 62-71% of patients 5, 1

For ruling in advanced fibrosis:

• FIB-4 >3.25 has a positive predictive value of 65% and specificity of 95-97% for advanced fibrosis 3, 1
• This threshold mandates complete hepatological workup 6

Intermediate zone (1.45-3.25):

• Requires additional non-invasive testing (such as transient elastography) or clinical correlation to determine fibrosis stage 6

Important Caveats

Age significantly impacts FIB-4 accuracy - the formula inherently increases with age, which can lead to false positives in elderly patients and false negatives in younger patients 1. In patients with rheumatoid arthritis on methotrexate, age did not correlate with FIB-4 values in the high FIB-4 group, suggesting the score performs differently in drug-induced liver injury 7.

The FIB-4 index is simple, free to calculate, and uses only routine laboratory tests, making it highly accessible for screening in primary care and resource-limited settings 2, 1. Online calculators are available (such as mdcalc.com) to facilitate rapid computation 8.