What is the formula to calculate the FIB-4 (Fibrosis-4) score?

How to Calculate FIB-4 Score

The FIB-4 score is calculated using the formula: Age (years) × AST (U/L) / [Platelets (10^9/L) × √ALT (U/L)] 1

Formula Breakdown

The FIB-4 calculation requires four parameters:

• Age in years
• AST (aspartate aminotransferase) in U/L
• ALT (alanine aminotransferase) in U/L
• Platelet count in 10^9/L

The steps to calculate FIB-4 are:

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

Interpretation of FIB-4 Results

FIB-4 scores are interpreted using established cutoff values:

• < 1.3: Low probability of advanced liver fibrosis (negative predictive value of 90%) 1, 2
• 1.3-2.67: Indeterminate range, may require additional testing 1
• > 2.67: High probability of advanced liver fibrosis (accuracy of 80%) 1
• > 3.25: Strongly suggestive of advanced fibrosis (positive predictive value of 65%) 1

Clinical Applications

FIB-4 is widely used for:

• Non-invasive assessment of liver fibrosis in chronic liver diseases including:
  • Chronic hepatitis B 1
  • Chronic hepatitis C 1
  • Non-alcoholic fatty liver disease (NAFLD) 1

Advantages and Limitations

Advantages:

• Simple calculation using routine laboratory parameters
• No additional cost beyond standard blood tests
• Validated in multiple populations 1
• Recommended by clinical practice guidelines as a first-line non-invasive test 1, 2

Limitations:

• Intermediate results may require additional testing such as transient elastography or liver biopsy 1
• Performance may vary between different liver diseases 1
• Less accurate than specialized imaging techniques like MR elastography 1

Clinical Recommendations

• FIB-4 should be used as an initial screening tool for liver fibrosis assessment 1, 2
• For indeterminate results, consider additional non-invasive tests like transient elastography 1
• Serial FIB-4 measurements can help monitor fibrosis progression over time 3
• A FIB-4 score < 1.3 reliably excludes advanced fibrosis with a negative predictive value ≥90% 2

Remember that while FIB-4 is a valuable non-invasive tool, liver biopsy remains the gold standard for definitive assessment of liver fibrosis in cases where non-invasive tests yield inconclusive results or when additional histological information is needed 1.