How to Calculate the FIB-4 Score
The FIB-4 score is calculated using a simple formula: Age (years) × AST (IU/L) / [Platelet count (×10⁹/L) × √ALT (IU/L)]. 1
Required Laboratory Values
You need exactly four values to calculate FIB-4: 1
- Age in years
- AST (aspartate aminotransferase) in IU/L
- ALT (alanine aminotransferase) in IU/L
- Platelet count in ×10⁹/L (or cells/mm³ divided by 1,000)
Practical Calculation Steps
Step 1: Multiply the patient's age by their AST level 2
Step 2: Take the square root of the ALT value 2
Step 3: Multiply the platelet count by the square root of ALT 2
Step 4: Divide the result from Step 1 by the result from Step 3 2
Many free online calculators are available (such as https://www.mdcalc.com/fibrosis-4-fib-4-index-liver-fibrosis) that perform this calculation automatically. 2
Interpreting Your Results
For Patients Under 65 Years Old:
- FIB-4 <1.3: Low probability of advanced fibrosis (negative predictive value ~90%) - repeat testing in 2-3 years 2
- FIB-4 1.3-2.67: Indeterminate risk - requires additional testing with liver stiffness measurement (LSM) by transient elastography or Enhanced Liver Fibrosis (ELF) blood test 2
- FIB-4 >2.67: High probability of advanced fibrosis - refer to hepatologist for comprehensive evaluation 2
For Patients 65 Years and Older:
Use a higher lower cutoff of 2.0 instead of 1.3 to rule out advanced fibrosis, as age significantly affects FIB-4 values and leads to higher false-positive rates in elderly patients. 2 The upper cutoff of 2.67 remains unchanged. 2
Special Considerations for Your Patient Population
Diabetes Impact on FIB-4 Accuracy:
FIB-4 performs less accurately in patients with diabetes, with a c-statistic of only 0.68 in diabetic patients compared to 0.85 in non-diabetic patients. 3 This means FIB-4 may miss some cases of advanced fibrosis in your patient with diabetes, obesity, and metabolic syndrome. 4
Obesity Considerations:
FIB-4 accuracy also varies by weight status, with reduced performance in normal weight individuals (c-statistic 0.65) compared to overweight patients (c-statistic 0.85). 3
What to Do After Calculating FIB-4
If FIB-4 is Low (<1.3 or <2.0 if age ≥65):
- Advanced fibrosis is unlikely 2
- Repeat FIB-4 testing every 2-3 years 2
- Continue management with primary care, dietician, endocrinologist for metabolic risk factors 2
If FIB-4 is Indeterminate (1.3-2.67):
- Proceed immediately to liver stiffness measurement (LSM) by vibration-controlled transient elastography (VCTE/FibroScan) 2
- If LSM <8 kPa: Low risk, repeat testing in 2-3 years 2
- If LSM 8-12 kPa: Indeterminate risk, refer to hepatologist for monitoring 2
- If LSM >12 kPa: High risk, refer to hepatologist for further evaluation 2
- If elastography unavailable, use Enhanced Liver Fibrosis (ELF) blood test as alternative (ELF <7.7 indicates low risk) 2
If FIB-4 is High (>2.67):
- Immediate referral to hepatologist is mandatory 2
- High probability of advanced fibrosis (positive predictive value 60-80%) 2, 1
- Hepatologist will perform additional testing (elastography, possible liver biopsy, or MR elastography) 2
Critical Pitfalls to Avoid
Do not use a second non-proprietary diagnostic panel (such as NAFLD fibrosis score) if FIB-4 is indeterminate, as these generally do not perform better than FIB-4 and add no additional value. 2 Instead, proceed directly to elastography or ELF testing.
FIB-4 has limited positive predictive value - a high score does not definitively confirm advanced fibrosis, only indicates high probability requiring further evaluation. 2, 1
Age is a major confounder - FIB-4 naturally increases with age independent of fibrosis, leading to false positives in elderly patients and potential false negatives in younger patients (<35 years). 2, 1
In patients with diabetes and metabolic syndrome, FIB-4 may underperform, so maintain a lower threshold for proceeding to elastography even with borderline scores. 4, 3