About the FIB-4 Calculator
The FIB-4 index is a simple blood-test score that estimates the likelihood of advanced liver fibrosis (scarring) without a biopsy. It uses four routine values — age, aspartate aminotransferase (AST), alanine aminotransferase (ALT) and platelet count — and was developed by Sterling and colleagues in 2006 in patients with HIV and hepatitis C co-infection. It has since become the recommended first-line test for fatty liver disease (MASLD, formerly NAFLD) in primary care.
Choose the cut-offs for your setting. For MASLD/NAFLD the American Association for the Study of Liver Diseases (AASLD) 2023 guidance uses below 1.3 as low risk, 1.3 to below 2.67 as indeterminate and 2.67 or more as high risk; in older adults (AASLD: over 65; McPherson’s derivation: 65 or over) the low-risk threshold rises to 2.0 because age inflates the score. For viral hepatitis the original Sterling cut-offs are below 1.45 and above 3.25.
A low FIB-4 has a high negative predictive value and is mainly used to rule out advanced fibrosis; an indeterminate or high value usually leads to a second test such as vibration-controlled transient elastography. FIB-4 is less accurate under age 35 and should not be used during acute illness or acute hepatitis.
With the default inputs, the fib-4 index is 1.69. Change any value above to recalculate instantly.
How to use the fib-4 calculator
- 1Enter age in years.
- 2Enter AST and ALT in U/L from the same liver panel.
- 3Enter the platelet count in ×10⁹/L (thousands per µL).
- 4Choose MASLD/NAFLD or viral hepatitis cut-offs.
- 5Read the FIB-4 value and risk band; indeterminate or high results usually need elastography.
Formula and method
FIB-4 rises with age and AST, and falls with platelet count and ALT. As fibrosis progresses, AST tends to rise relative to ALT and platelets fall (from portal hypertension and reduced thrombopoietin), so the ratio captures several signals of advancing disease. The square root of ALT dampens the effect of ALT.
For MASLD/NAFLD, AASLD 2023 guidance uses < 1.3 (low risk), 1.3 to < 2.67 (indeterminate) and ≥ 2.67 (high risk), with a rule-out threshold of 2.0 in older adults: McPherson 2017 derived it for age ≥ 65, raising specificity from 35% to 70% with 77% sensitivity. This calculator applies it from age 65. Sterling’s original HIV/HCV study used < 1.45 (negative predictive value 90%, sensitivity 70%) and > 3.25 (positive predictive value 65%, specificity 97%).
- AST
- Aspartate aminotransferase (U/L)
- ALT
- Alanine aminotransferase (U/L)
- Platelets
- Platelet count (10⁹/L = 10³/µL)
Worked examples
Indeterminate result in fatty liver disease
(50 × 40) ÷ (200 × √35) = 2000 ÷ 1183.2 = 1.69. That lies between 1.3 and 2.67, so advanced fibrosis cannot be ruled in or out and a second test such as elastography is usually advised.
Low-risk result
(45 × 25) ÷ (280 × √30) = 1125 ÷ 1533.6 = 0.73, below 1.3, making advanced fibrosis unlikely.
High-risk result
(68 × 60) ÷ (150 × √40) = 4080 ÷ 948.7 = 4.30, above 2.67, indicating a high likelihood of advanced fibrosis.
Older adult using the age-adjusted threshold
(70 × 30) ÷ (220 × 5) = 1.91. This would be indeterminate under the standard 1.3 cut-off, but for people aged 65 or over the rule-out threshold is 2.0, so it counts as low risk.
Frequently asked questions
What is a normal FIB-4 score?+
For fatty liver disease, a FIB-4 below 1.3 (below 2.0 if aged 65 or over) is considered low risk for advanced fibrosis. Values from 1.3 up to 2.67 are indeterminate and 2.67 or more indicate high risk. For viral hepatitis the cut-offs are 1.45 and 3.25.
Why is there a different cut-off for older people?+
Age is part of the formula, so FIB-4 rises with age even without liver disease. McPherson and colleagues showed specificity for advanced fibrosis fell to 35% at age 65 and over; a rule-out threshold of 2.0 raised it to 70% without losing sensitivity, and AASLD guidance adopts a 2.0 cut-off for people over 65.
What happens after an indeterminate or high FIB-4?+
Guidelines usually recommend a second non-invasive test, most often vibration-controlled transient elastography (FibroScan) or the ELF blood test, and referral to hepatology for high-risk results.
Can FIB-4 be used in under-35s?+
FIB-4 has low accuracy below age 35 because the age term keeps the score low. AASLD suggests considering a secondary assessment in younger people with metabolic risk factors or raised liver enzymes.
What units should platelets be in?+
Enter platelets in ×10⁹/L, which is numerically the same as thousands per microlitre (×10³/µL). A count of 200,000/µL is entered as 200.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.
Sources
- Sterling RK et al. Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection. Hepatology 2006;43:1317–25 (PubMed)
- Rinella ME et al. AASLD Practice Guidance on the clinical assessment and management of nonalcoholic fatty liver disease. Hepatology 2023 (PMC)
- McPherson S et al. Age as a confounding factor for the accurate non-invasive diagnosis of advanced NAFLD fibrosis. Am J Gastroenterol 2017 (PMC)