FIB-4 Index Calculator

How it is calculated

The index combines age with three routine blood tests: FIB-4 = (age × AST) ÷ (platelets × √ALT). Age is in years, AST and ALT are in U/L, and platelets are in 10⁹/L (thousands per microlitre). The tool needs all four values and returns nothing if platelets or ALT are zero or negative. The result is rounded to two decimals. Commonly cited cut-offs are about 1.3 (low risk) and 2.67 (high risk), with the values in between left uncertain, but these depend on age and clinical setting, so the number must be interpreted by a clinician. It is a screening aid, not a diagnosis.

Common mistakes

• Entering platelets as a raw count such as 200,000 instead of 200 (10⁹/L). The result then comes out about a thousand times too small.
• Swapping AST and ALT. Only ALT sits under the square root, so reversing them changes the score.
• Using lab values in different units or from different days and expecting a meaningful score; the three tests should come from the same blood draw.
• Treating the number as a diagnosis. A value near a cut-off, or one measured in a younger or older patient, needs a doctor's judgment and often further testing.