FIB-4 Index Calculator
Understanding the FIB-4 Index
The FIB-4 (Fibrosis-4) index is a non-invasive, simple, and widely used scoring system to assess the likelihood of advanced liver fibrosis (scarring) in patients with chronic liver diseases, particularly those with non-alcoholic fatty liver disease (NAFLD) or chronic hepatitis C.
Why is FIB-4 Used?
Liver biopsy is the gold standard for diagnosing and staging liver fibrosis, but it is an invasive procedure with potential risks. The FIB-4 index offers a convenient and cost-effective alternative for initial screening, helping clinicians identify patients who may have advanced fibrosis and require further evaluation, or those who are at low risk and may not need immediate invasive procedures.
How is FIB-4 Calculated?
The FIB-4 index is calculated using four readily available clinical parameters:
- Age: The patient's age in years.
- AST (Aspartate Aminotransferase): A liver enzyme level, typically measured in Units per Liter (U/L).
- ALT (Alanine Aminotransferase): Another liver enzyme level, also measured in Units per Liter (U/L).
- Platelet Count: The number of platelets in the blood, usually expressed as x 103/µL (or Giga/L).
The formula for the FIB-4 index is:
FIB-4 = (Age * AST) / (Platelets * √ALT)
Where:
- Age is in years.
- AST and ALT are in U/L.
- Platelets are in x 103/µL.
Interpreting Your FIB-4 Score
The FIB-4 score helps categorize the risk of advanced fibrosis (stages F3-F4). The interpretation thresholds can vary slightly based on guidelines and patient populations (e.g., HIV/HBV co-infection), but common thresholds for the general population are:
- FIB-4 < 1.30: This generally indicates a low probability of advanced liver fibrosis. In many cases, further invasive testing may not be immediately necessary.
- FIB-4 between 1.30 and 2.67: This is considered an indeterminate zone. The risk of advanced fibrosis is neither low nor high. Patients in this range may require additional non-invasive tests (e.g., elastography) or clinical assessment to determine the need for further evaluation.
- FIB-4 > 2.67: This suggests a high probability of advanced liver fibrosis. These patients are often recommended for further evaluation, which may include elastography or liver biopsy, to confirm the diagnosis and stage of fibrosis.
Important Note: The FIB-4 index is a screening tool, not a definitive diagnostic test. It should always be interpreted in conjunction with a patient's full clinical picture, medical history, and other diagnostic findings by a qualified healthcare professional. It is not validated for all patient groups (e.g., very young or very old patients).
Example Calculation:
Let's consider a 50-year-old patient with the following lab results:
- Age: 50 years
- AST: 45 U/L
- ALT: 30 U/L
- Platelet Count: 200 x 103/µL
Using the formula:
FIB-4 = (50 * 45) / (200 * √30)
FIB-4 = 2250 / (200 * 5.477)
FIB-4 = 2250 / 1095.4
FIB-4 ≈ 2.05
In this example, a FIB-4 score of approximately 2.05 falls into the indeterminate zone (between 1.30 and 2.67), suggesting that further evaluation might be warranted.