How to use this calculator
Gather the following values from a recent blood test:
- Age — Your current age in years.
- AST (Aspartate Transaminase) — Measured in U/L, also labelled AspAT, ASAT, GOT, or SGOT.
- ALT (Alanine Transaminase) — Measured in U/L, also labelled ALAT or SGPT.
- Platelet count — Expressed as cells per mm³, per μL, or × 10⁹/L (your lab report will specify the unit).
Enter each value into the corresponding field. The calculator will compute your FIB-4 score and APRI ratio instantly. This tool is most reliable for people aged 35–65 years; results outside this age range may be less accurate or produce false positives.
FIB-4 and APRI formulas
The FIB-4 index combines age, liver enzymes, and platelet count into a single dimensionless score. The APRI ratio uses AST and platelets to estimate fibrosis severity in hepatitis C and other conditions. Both avoid the need for biopsy.
FIB-4 = (Age × AST) ÷ (Platelet count × √ALT)
APRI = ((AST ÷ 40) ÷ Platelet count) × 100
Age— Patient age in years.AST— Aspartate transaminase level in U/L.ALT— Alanine transaminase level in U/L.Platelet count— Number of platelets, typically expressed as 10⁹/L or × 10⁹/L.
Interpreting your FIB-4 score
FIB-4 < 1.45 — Suggests absence of advanced fibrosis (F3–F4) with ~95% confidence. Reassuring result; routine monitoring usually sufficient.
FIB-4 between 1.45 and 3.25 — Intermediate risk. Further evaluation (imaging, specialist referral) may be warranted, depending on clinical context.
FIB-4 ≥ 3.25 — Indicates probable advanced fibrosis (F3–F4) with ~82% confidence. Warrants urgent assessment and possible antiviral or disease-modifying therapy.
Remember: FIB-4 is a screening tool, not diagnostic. A single score does not replace clinical judgment, imaging (ultrasound, elastography), or biopsy if uncertainty remains.
APRI score interpretation and use
The APRI (AST to Platelet Ratio Index) was originally developed to assess cirrhosis risk in patients with chronic hepatitis C, but is also applied to other liver diseases.
APRI < 0.5 — Low likelihood of cirrhosis or severe fibrosis. Disease progression is unlikely.
APRI 0.5 to 1.0 — Intermediate risk. Clinical assessment and imaging may clarify disease stage.
APRI ≥ 1.0 — High probability of advanced fibrosis or cirrhosis. Immediate specialist review and treatment planning recommended.
APRI is particularly useful in hepatitis C cohorts and can complement FIB-4 for a more complete liver fibrosis risk profile.
Key considerations when using this calculator
Accurate results depend on correct blood values and awareness of the calculator's limitations.
- Age matters for FIB-4 accuracy — The FIB-4 formula was validated in adults aged 35–65 years. Younger or older patients may receive inaccurate scores. If you fall outside this range, discuss results with your clinician.
- Platelet count as a fibrosis proxy — Low platelet counts often reflect portal hypertension or bone marrow effects of liver disease. However, thrombocytopenia from other causes (splenomegaly, autoimmune conditions, medications) can produce falsely high FIB-4 scores.
- Normal lab variation and timing — AST and ALT fluctuate with viral load, alcohol use, medications, and exercise. Obtain fasting samples and repeat testing if results are borderline or unexpected. A single result should not dictate treatment decisions.
- FIB-4 complements, does not replace, imaging — Transient elastography (FibroScan) or ultrasound can confirm suspected fibrosis. If FIB-4 is discordant with imaging or clinical signs, seek specialist review rather than relying on one tool alone.