The Fibrosis-4 (FIB-4) index is a simple, non-invasive tool used to estimate liver scarring (fibrosis). This score combines routine blood work results with a patient’s age to assess liver health. Initially developed for chronic hepatitis, it is now widely used to screen individuals with chronic liver conditions, particularly Non-Alcoholic Fatty Liver Disease (NAFLD), often called Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD). The FIB-4 score determines an individual’s risk of having advanced liver fibrosis (F3–F4), helping doctors decide if more intensive testing is needed. It provides risk stratification rather than a definitive diagnosis.
The Four Variables Used in Calculation
The calculation of the FIB-4 index requires four readily available clinical parameters routinely collected during standard blood tests. These variables are combined using a specific mathematical formula to generate the final numerical score.
The first factor is the patient’s age in years. Age is a significant component because the risk of developing advanced liver fibrosis naturally increases over time, even with relatively stable underlying liver disease. Therefore, older individuals are more likely to generate a higher FIB-4 score, reflecting this increased baseline risk.
The next two components are the liver enzymes: Aspartate Aminotransferase (AST) and Alanine Aminotransferase (ALT). Both are enzymes found in liver cells, and elevated levels in the bloodstream typically indicate that the liver cells are damaged or inflamed. While ALT is often considered a more specific marker for liver injury, the ratio between AST and ALT provides a more nuanced view of the type and severity of the liver damage.
The final variable needed for the calculation is the platelet count. Platelets are small blood cells involved in clotting, but a lower-than-normal count, known as thrombocytopenia, can be an indirect sign of advanced liver disease. In severe liver scarring, blood flow through the liver can be impaired, which may cause the spleen to lower the platelet count in the circulating blood.
Interpreting the Risk Thresholds
The FIB-4 score does not have a single “normal range” like a fixed blood test reference range. Instead, the score is interpreted using specific numerical thresholds that divide results into three risk categories: low, intermediate, and high probability of advanced liver fibrosis. The most commonly used cut-off values for assessing advanced fibrosis (F3-F4) in patients with NAFLD/MASLD are 1.30 and 2.67.
The low-risk category is defined by a FIB-4 score of less than 1.30. A score below this threshold suggests a very low probability of advanced fibrosis, often having a negative predictive value of over 90%. This result typically indicates minimal or no significant scarring (F0-F1) and allows for management in a primary care setting.
The high-risk category includes scores greater than 2.67. This elevated score indicates a high probability of advanced fibrosis or even cirrhosis. The positive predictive value for advanced fibrosis in this range often falls between 60% and 80%. For individuals over the age of 65, some guidelines use a lower high-risk threshold, such as 2.0, due to the natural age-related increase in the score.
The intermediate, or indeterminate, risk category falls between the two main cut-offs (1.30 to 2.67). A result in this range is inconclusive and does not reliably confirm or rule out advanced fibrosis. Further diagnostic testing is generally recommended to clarify the actual stage of liver scarring.
Clinical Follow-Up Based on Results
The FIB-4 score is primarily a screening tool that guides the next steps in patient care. The resulting risk stratification determines the urgency and type of subsequent evaluation needed.
For individuals in the low-risk category (FIB-4 < 1.30), the immediate concern for advanced fibrosis is minimal. Follow-up usually focuses on continued monitoring of underlying liver disease risk factors and recommending therapeutic lifestyle changes. This often involves advising on diet modifications, increased physical activity, and weight management, with the FIB-4 score potentially being re-evaluated every two to three years. When the FIB-4 score indicates a high risk (FIB-4 > 2.67), it signals the need for prompt referral to a liver specialist, often without requiring additional preliminary testing. The specialist will likely use confirmatory tests to determine the precise stage of fibrosis and to check for complications. These next steps typically involve non-invasive imaging techniques, such as transient elastography (FibroScan), which measures liver stiffness.
For patients whose score falls within the indeterminate range (1.30–2.67), further investigation is necessary to resolve the uncertainty. This often involves a second-line, non-invasive test, such as transient elastography or the Enhanced Liver Fibrosis (ELF) test, to more accurately stage the fibrosis. In rare cases, a liver biopsy may be considered to finalize the diagnosis and guide management, as it remains the gold standard for staging fibrosis.