How this calculator works
FIB-4 combines age, AST, ALT and platelets. The original study used adults with HIV/HCV co-infection, with cut-offs 1.45 and 3.25. Other diseases and age groups use different pathways; these original thresholds should not be substituted for a current MASLD protocol. Acute illness, low platelets from other causes and age can distort interpretation. Reverse modes show algebraically implied values rather than measured results. The result is an estimate for information only and is not medical advice.
Formula
FIB-4 = age(years) x AST(U/L) / [platelets(10^9/L) x square root of ALT(U/L)]. All denominator inputs must be positive.
Worked example
Age 50, AST 80, ALT 100 and platelets 200 give FIB-4 = 4000 / 2000 = 2. Age 40, AST 30, ALT 25 and platelets 240 give 1.
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Frequently asked questions
Should I use 1.45 and 3.25 for every liver condition?
No. They are the original cohort thresholds; current disease-specific guidance may use different boundaries.
Can I enter platelets as 200000?
Only if the tool asked for cells per microlitre, which it does not. Enter 200 for 200 x 10^9/L.
Can I solve for a missing laboratory value?
The algebra returns an implied value for a chosen score. Clinical assessment requires actual measurements.