FIB-4 Calculator
Estimate liver fibrosis risk using the FIB-4 score from age, AST, ALT, and platelet count.
What Is the FIB-4 Score?
The FIB-4 (Fibrosis-4) index is a non-invasive scoring system used to estimate the degree of liver fibrosis in patients with chronic liver disease. It combines four routine laboratory values and age to calculate a score that correlates with the likelihood of advanced fibrosis or cirrhosis.
Developed as an alternative to liver biopsy, the FIB-4 index helps clinicians identify patients who may need further evaluation without exposing them to invasive procedures. The score is particularly validated in patients with non-alcoholic fatty liver disease (NAFLD), hepatitis B, and hepatitis C.
How the FIB-4 Score Is Calculated
The formula uses age, AST (aspartate aminotransferase), ALT (alanine aminotransferase), and platelet count:
FIB-4 = (Age × AST) / (Platelet Count × √ALT)
Where:
- Age is in years
- AST and ALT are in U/L
- Platelet Count is in 10⁹/L
The square root of ALT in the denominator means that disproportionately elevated ALT relative to AST can lower the score, while older age and higher AST increase it.
Interpreting FIB-4 Results
The FIB-4 score is typically interpreted using established cut-off values:
| FIB-4 Score | Interpretation |
|---|---|
| < 1.45 | Low risk of advanced fibrosis |
| 1.45 – 3.25 | Indeterminate risk — further evaluation recommended |
| > 3.25 | High risk of advanced fibrosis |
These thresholds have high negative predictive value at the low end and high positive predictive value at the high end. The indeterminate range (1.45–3.25) is where additional testing such as elastography or biopsy may be most useful.
Important Considerations
- The FIB-4 index is a screening tool, not a diagnostic test. A low score does not completely rule out fibrosis, and a high score does not confirm cirrhosis.
- Results should always be interpreted in the context of the patient's full clinical picture, including etiology of liver disease, symptoms, and other laboratory findings.
- Age is a significant factor in the formula. In older patients, the score may be elevated even without significant fibrosis, which is why some adjusted cut-offs have been proposed for patients over 65.
Common Mistakes When Using the FIB-4 Calculator
- Using incorrect units: Platelet count must be entered in 10⁹/L (not thousands/µL). If your lab reports platelets as 150,000/µL, that equals 150 × 10⁹/L.
- Entering AST and ALT in the wrong order: The formula places AST in the numerator and ALT under the square root. Swapping them produces an incorrect result.
- Applying the score to acute liver injury: FIB-4 is validated for chronic liver disease. In acute hepatitis, transaminases can be transiently elevated, making the score unreliable.
- Ignoring the indeterminate range: A score of 2.0 does not mean moderate fibrosis — it means the result is inconclusive and requires further investigation.
Practical Use Cases
- Primary care screening: Identify patients with NAFLD who may benefit from referral to a hepatologist.
- Monitoring disease progression: Track FIB-4 over time in patients with known chronic liver disease to detect worsening fibrosis.
- Pre-procedural risk assessment: Evaluate liver fibrosis risk before surgeries or procedures that may be affected by liver function.
- Research and epidemiology: Estimate fibrosis prevalence in population studies without invasive testing.
Limitations of the FIB-4 Index
- Less accurate in patients under 35 or over 65, where age weighting may skew results.
- Not validated for all causes of liver disease — most evidence supports use in NAFLD, hepatitis B, and hepatitis C.
- Can be affected by conditions that alter platelet counts independently of liver disease, such as immune thrombocytopenia or myelodysplastic syndromes.
- The indeterminate range (1.45–3.25) captures a significant proportion of patients, limiting the test's ability to provide a definitive answer in many cases.
FAQ
What does FIB-4 stand for?
FIB-4 stands for Fibrosis-4, referring to the four variables used in the calculation: age, AST, ALT, and platelet count.
Is FIB-4 better than AST/ALT ratio?
FIB-4 generally has better diagnostic accuracy for advanced fibrosis than the simple AST/ALT ratio, particularly in NAFLD patients. It incorporates age and platelet count, which are independently associated with fibrosis severity.
Can FIB-4 replace a liver biopsy?
No. FIB-4 is a screening tool that helps determine who needs further evaluation. It can reduce the number of unnecessary biopsies but cannot replace histologic assessment when definitive staging is required.
What if my FIB-4 score is in the indeterminate range?
An indeterminate score (1.45–3.25) means the test is inconclusive. Your doctor may recommend additional non-invasive tests such as transient elastography (FibroScan), MRI elastography, or a liver biopsy to clarify the degree of fibrosis.
How often should FIB-4 be checked?
For patients with chronic liver disease, FIB-4 is typically rechecked annually or as clinically indicated. More frequent monitoring may be appropriate if there is concern for rapid progression.