APRI Calculator (AST to Platelet Ratio Index)
APRI Calculator (AST to Platelet Ratio Index)
Estimate liver fibrosis from AST and platelet count alone — the simplest of the non-invasive fibrosis indices.
APRI (AST to Platelet Ratio Index)
AST + plateletsAST 78 U/L, ULN 40 U/L, platelets 140 ×10⁹/L
Formula
- AST
- U/L
- AST ULN
- your laboratory's upper reference limit, commonly 40 U/L — the index is sensitive to this choice
- Platelets
- ×10⁹/L, equivalent to ×10³/µL
Worked example
AST 78 U/L, ULN 40 U/L, platelets 140 ×10⁹/L
78 ÷ 40 = 1.95
(1.95 ÷ 140) × 100 = 1.39
Between 1.0 and 1.5 → significant fibrosis likely
Thresholds
| APRI | Interpretation |
|---|---|
| < 0.5 | Significant fibrosis unlikely |
| 0.5 – 1.0 | Indeterminate |
| 1.0 – 1.5 | Significant fibrosis likely |
| > 1.5 | Cirrhosis likely |
Where APRI fits
APRI was derived in chronic hepatitis C and needs only two values that appear on almost every routine blood panel. That simplicity is its point: it can be calculated retrospectively from existing results, at no cost, anywhere. The World Health Organization recommends it for fibrosis staging in hepatitis B where elastography is unavailable.
The physiology behind it is straightforward. AST rises with hepatocyte injury and, as fibrosis progresses, rises relative to ALT. Platelets fall as portal hypertension develops and the spleen sequesters them, and as a failing liver makes less thrombopoietin. A rising numerator over a falling denominator makes the ratio climb steeply in advanced disease.
Its accuracy is modest compared with FIB-4 or elastography, and it performs best at the extremes. The dependence on the local AST upper limit of normal is an underappreciated weakness — the same patient scores differently in two laboratories using different reference limits, so quote the ULN used alongside the result.
Frequently asked questions
What is a normal APRI score?
Below 0.5 makes significant fibrosis unlikely. Above 1.5 suggests cirrhosis. Between those values the result is indeterminate and needs a second test.
Which AST upper limit of normal should I use?
Your reporting laboratory’s own. It commonly sits around 40 U/L but varies with method and reference population, and the index is directly sensitive to the value chosen.
Is APRI better than FIB-4?
Generally no. FIB-4 includes age and ALT and performs better in fatty liver disease. APRI’s advantage is that it needs only two values and no age, which suits retrospective use and resource-limited settings.
Can APRI replace a biopsy?
No. It is a triage test. A low score reliably avoids further fibrosis work-up; a raised one is a reason for elastography or specialist referral rather than a diagnosis.
Related calculators
References
- Wai CT et al. A simple noninvasive index can predict both significant fibrosis and cirrhosis in patients with chronic hepatitis C. Hepatology. 2003;38(2):518–26.
- WHO. Guidelines for the prevention, care and treatment of persons with chronic hepatitis B infection. 2015.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.