How do I calculate the Fibrosis‑4 (FIB‑4) index for a patient with hepatomegaly, diffuse fatty infiltration on imaging, and mildly elevated alanine aminotransferase (ALT)?

How to Calculate the FIB-4 Index

The FIB-4 index is calculated using the formula: FIB-4 = (age in years × AST in U/L) / (platelet count in 10⁹/L × √ALT in U/L) 1.

Required Laboratory Values

To calculate FIB-4, you need four simple parameters:

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

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

Interpretation Thresholds

For patients <65 years old 1, 2:

• FIB-4 <1.3: Low risk of advanced fibrosis (negative predictive value ≥90%)
• FIB-4 1.3-2.67: Indeterminate range requiring further evaluation with elastography or enhanced liver fibrosis (ELF) testing
• FIB-4 >2.67: High risk of advanced fibrosis (positive predictive value 60-80%), warrants hepatology referral

For patients ≥65 years old 1:

• FIB-4 <2.0: Low risk of advanced fibrosis (use this higher threshold instead of 1.3)
• FIB-4 >2.0: Increased risk requiring further assessment

Clinical Context for Your Patient

For your patient with hepatomegaly, fatty infiltration, and mildly elevated ALT, the FIB-4 serves as an excellent first-line screening tool to assess fibrosis risk in the setting of suspected metabolic dysfunction-associated steatotic liver disease (MASLD) 1, 2. This calculation requires only routine laboratory tests and provides immediate risk stratification without additional cost.

Important Caveats

FIB-4 has significant limitations 1:

• Limited accuracy in the indeterminate range (1.3-2.67), where 30-40% of patients fall
• Reduced performance in elderly patients and those with type 2 diabetes
• Can produce false positives in low-prevalence populations
• Will miss approximately 10% of individuals with advanced fibrosis even when below the low-risk threshold
• Recent evidence shows that diabetes and obesity significantly reduce FIB-4 accuracy 3—patients with these conditions may need elastography even with FIB-4 <1.3

Age-related considerations 1, 4:

• The standard cutoff of <1.3 applies only to those under 65 years
• For those over 65 years, use the modified lower cutoff of <2.0 to avoid excessive false positives
• FIB-4 performs poorly and should not be used in individuals younger than 35 years 1

Next Steps After Calculation

If FIB-4 is in the indeterminate range (1.3-2.67), two evidence-based options exist 1:

1. Proceed directly to liver elastography (vibration-controlled transient elastography/FibroScan) to clarify fibrosis stage, particularly if FIB-4 is closer to 2.67
2. Implement 1 year of lifestyle intervention and cardiometabolic risk factor management, then retest FIB-4; if still elevated, proceed to elastography

For patients with diabetes or BMI ≥30 kg/m², consider fast-tracking to elastography even with FIB-4 <1.3, as these conditions materially increase the probability of advanced fibrosis at any given FIB-4 value 3.