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FIB-4 Index Calculator

Find the FIB-4 index.

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.

Worked example

Take a 55-year-old with AST 40, ALT 36 and platelets 200. Numerator: 55 × 40 = 2,200. Denominator: the square root of 36 is 6, and 200 × 6 = 1,200. Then 2,200 ÷ 1,200 = 1.83. The FIB-4 index is 1.83, which falls between the two commonly used cut-offs.

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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.

Sources

About this calculator

This calculator computes the FIB-4 index using FIB-4 = (Age × AST) ÷ (Platelet count × √ALT), a formula that combines a patient's age with three routine lab values — AST, ALT, and platelet count — into a single score.

Hepatologists, gastroenterologists, and primary care clinicians use it as a non-invasive first-line screen for liver fibrosis in patients with chronic hepatitis B or C, or non-alcoholic fatty liver disease (NAFLD/NASH), helping decide who needs further workup like elastography or biopsy versus who can be monitored routinely.

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Comments (1)

  • Editorial team

    The inputs are the standard ones: age in years, AST and ALT in U/L, and platelets in thousands per microliter, which is the same as 10^9 per litre. A platelet count typed as 200000 instead of 200 will give a tiny index that means nothing. The cut-offs used to read the score vary with age, so the number alone is not a diagnosis; discuss it with the doctor who ordered the tests. Which lab units appear on your report?

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