Hepatology
APRI (AST–Platelet Ratio)
Non-invasive estimate of liver fibrosis.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The AST-to-Platelet Ratio Index (APRI) is a simple, non-invasive marker of liver fibrosis that uses only the aspartate aminotransferase (AST) level and the platelet count. It is calculated as (AST / AST upper limit of normal) × 100, divided by the platelet count in ×10⁹/L. The rationale is that fibrosis tends to raise AST (reflecting hepatocellular injury) and lower platelets (reflecting portal hypertension and reduced thrombopoietin), so their ratio rises as fibrosis advances. It was developed primarily in chronic hepatitis C as an alternative to liver biopsy.
Interpreting the result
Lower values make significant fibrosis unlikely and higher values make it likely: a value below about 0.5 suggests significant fibrosis is improbable, 0.5–1.5 is indeterminate, and a value above about 1.5 suggests significant fibrosis or cirrhosis. The score is best used to rule fibrosis in or out at its extremes, with the indeterminate middle band often requiring further assessment. Because it was derived in viral hepatitis, performance in other liver diseases is more variable.
Worked example
A patient with chronic hepatitis C has an AST of 80 U/L (laboratory upper limit of normal 40 U/L) and a platelet count of 100 ×10⁹/L. APRI = (80/40) × 100 / 100 = 2.0, which is above 1.5 and suggests significant fibrosis or cirrhosis is likely.
Critical actions
APRI = (AST / AST-ULN) × 100 / platelets (×10⁹/L). Use your lab's AST upper limit of normal.
Pearls / pitfalls
- Always use your own laboratory's AST upper limit of normal, as this directly scales the result.
- AST can rise from non-hepatic sources (muscle injury, haemolysis) and is also affected by alcohol, which can distort the index.
- It is most reliable in chronic viral hepatitis; performance in fatty liver disease and other causes is less consistent.
- The wide indeterminate zone (0.5–1.5) is common, so pair APRI with FIB-4 or elastography rather than relying on it alone.
Evidence & validation
Derived and validated by Wai and colleagues in chronic hepatitis C, APRI is endorsed by the World Health Organization for staging fibrosis in resource-limited settings and is included in hepatitis guidance as a low-cost non-invasive test, often used alongside FIB-4.
Frequently asked questions
How is APRI calculated?
APRI = (AST / AST upper limit of normal) × 100, divided by the platelet count in ×10⁹/L. Use your laboratory's own AST upper limit of normal, as it scales the result directly.
What APRI value suggests significant fibrosis?
A value above about 1.5 suggests significant fibrosis or cirrhosis is likely, while a value below about 0.5 makes it unlikely. Values in between are indeterminate and usually need further testing.
Is APRI accurate in fatty liver disease?
It was derived in chronic hepatitis C and performs best there. In non-alcoholic fatty liver disease and other causes its accuracy is more variable, so scores such as the NAFLD fibrosis score or FIB-4 are often preferred.
Can APRI replace a liver biopsy?
It can reduce the need for biopsy by ruling fibrosis in or out at the extremes, but the large indeterminate band and dependence on AST mean it is best combined with other non-invasive tests rather than used alone.
What can falsely raise APRI?
A high AST from non-liver sources such as muscle injury or haemolysis, or a low platelet count from other causes, can raise the index independently of fibrosis, so interpret it in clinical context.
References
- Wai CT, Greenson JK, Fontana RJ, et al. A simple noninvasive index can predict both significant fibrosis and cirrhosis in patients with chronic hepatitis C. Hepatology. 2003;38(2):518–526.
- World Health Organization. Guidelines for the screening, care and treatment of persons with chronic hepatitis C infection. 2018.
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