FIB-4 Index Calculator (Liver Fibrosis Score)

The FIB-4 (Fibrosis-4) Index is a non-invasive score that estimates the likelihood of advanced liver fibrosis (METAVIR stage F3–F4) from values already on a routine blood panel. It was derived by Sterling et al., 2006 and is now recommended by the AASLD (2023) and the AGA (2021) as the first-line test to decide who, among people with fatty liver disease or metabolic risk factors, needs a closer look — without a biopsy.

The formula is:

FIB-4 = (Age × AST) ÷ (Platelets × √ALT)

Age is in years, AST and ALT in U/L (IU/L), and the platelet count in ×10⁹/L (the same number as thousand per µL — a report reading 150,000/µL is entered as 150). Because the denominator contains the platelet count and the square root of ALT, both must be greater than zero; the calculator rejects a zero or blank value with a specific message rather than showing a misleading result.

The result is read against two cut-offs from the AGA 2021 Clinical Care Pathway. A FIB-4 below the lower cut-off is low risk — its high negative predictive value reasonably excludes advanced fibrosis. Between the two cut-offs is indeterminate, where a second-line test such as transient elastography (FibroScan) or the ELF blood test is advised. Above 2.67 is high risk, supporting referral and further evaluation.

One age adjustment matters. For adults under 65 the lower cut-off is 1.30. For patients aged 65 or older it is raised to 2.00, because the 1.30 threshold produces too many false positives in older people; the upper 2.67 cut-off is unchanged at every age. This tool applies the correct lower cut-off automatically and states which one it used. To guard against a coding slip, it computes the index twice — once as written and once via the rearranged form (Age ÷ Platelets) × (AST ÷ √ALT) — and shows the second as a cross-check. FIB-4 does not diagnose fibrosis and does not replace clinical judgement, a hepatologist or a biopsy.

• Sterling RK, Lissen E, Clumeck N, et al. Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection. Hepatology 2006;43(6):1317-1325.
• Kanwal F, Shubrook JH, Adams LA, et al. AGA Clinical Care Pathway for the Risk Stratification and Management of Patients With NAFLD. Gastroenterology 2021;161(5):1657-1669.
• Rinella ME, Neuschwander-Tetri BA, Siddiqui MS, et al. AASLD Practice Guidance on the clinical assessment and management of MASLD. Hepatology 2023;77(5):1797-1835.

The formula and the cut-offs, including the age-65 adjustment, were last cross-checked against these sources on 2026-07-16. The FIB-4 formula has never been revised.