Medical AIAST to Platelet Ratio Index (APRI) for liver fibrosis
A non-invasive index using AST (relative to its upper limit of normal) and platelet count to estimate liver fibrosis. WHO hepatitis B guidance uses APRI greater than 2 to identify cirrhosis in adults in resource-limited settings; lower cutoffs (around 0.5 and 1.5) are used to rule out or rule in significant fibrosis, with the usual sensitivity and specificity trade-offs.
How to measure each input
- AST
- Aspartate aminotransferase in U/L from a recent sample. Acute hepatocellular injury (for example a flare or alcohol binge) transiently raises AST and inflates APRI, so interpret in clinical context.
- AST upper limit of normal
- Use your laboratory's stated AST ULN. WHO and most validation studies use 40 U/L. Because APRI scales inversely with the ULN, choosing a different ULN changes the result, so the same ULN must be used consistently.
- Platelet count
- Platelet count in 10^9/L (equivalently 1000 per microL). A count of 120,000/microL is entered as 120. Thrombocytopenia from non-hepatic causes (for example marrow disease or hypersplenism unrelated to portal hypertension) will affect the index.
Interpretation
| Band | Meaning |
|---|---|
| <= 0.5, cirrhosis and significant fibrosis unlikely | An APRI of 0.5 or below has good negative predictive value for ruling out cirrhosis and makes significant fibrosis less likely. APRI is most useful at its extremes; confirm with clinical context and, where available, elastography. |
| 0.5 to 1.5, indeterminate | An intermediate APRI is non-diagnostic. Fibrosis is neither confidently excluded nor confirmed. Consider a second non-invasive test (for example FIB-4 or transient elastography) or specialist assessment. |
| 1.5 to 2, significant fibrosis more likely | An APRI above 1.5 has greater positive predictive value for significant fibrosis. It does not by itself confirm cirrhosis, for which WHO uses a higher cutoff of greater than 2. |
| > 2, cirrhosis likely (WHO hepatitis B cutoff) | WHO hepatitis B guidance uses an APRI greater than 2 to identify cirrhosis in adults where biopsy and elastography are unavailable. This cutoff is specific (around 91%) but insensitive (around 46%), so a value of 2 or below does not exclude cirrhosis. Treat and stage per guidelines. |
Pitfalls, exclusions and caveats
- APRI is a probabilistic index, not a diagnosis. It performs best at its extremes; intermediate values are common and uninformative.
- The AST ULN you choose materially changes the result. WHO uses 40 U/L; using a lower or sex-specific ULN shifts every APRI value.
- It was derived in chronic hepatitis C (Wai 2003) and is endorsed by WHO for hepatitis B. Diagnostic accuracy is lower in other causes such as MASLD, so combine with other tests.
- Acute AST elevation (alcohol, drug injury, hepatitis flare) transiently raises APRI and overstates fibrosis.
- Thrombocytopenia from causes other than portal hypertension, and conditions raising AST without fibrosis, both distort the index.
APRI = ((AST / AST upper limit of normal) / platelet count) * 100, with platelet count in 10^9/L. AST and its ULN in the same units (U/L); the ULN cancels the AST units, so APRI is dimensionless.APRI was described by Wai et al. (2003) and adopted in WHO hepatitis B guidance. This implementation is an educational tool and is not affiliated with the original authors or any guideline body.
Frequently asked
What APRI value suggests cirrhosis?
WHO hepatitis B guidance uses an APRI greater than 2 to identify cirrhosis in adults where biopsy and elastography are unavailable. This cutoff is specific (around 91%) but insensitive (around 46%), so an APRI of 2 or below does not rule cirrhosis out.
Which AST upper limit of normal should I use?
WHO and most validation studies use 40 U/L, which is the default here. Because APRI scales inversely with the ULN, using a different value changes every result, so use a consistent, laboratory-appropriate ULN.
Is APRI accurate in MASLD or alcohol-related liver disease?
APRI was derived in chronic hepatitis C and is endorsed by WHO for hepatitis B. Its accuracy is lower in MASLD and other causes, and acute AST elevation overstates fibrosis, so combine it with other non-invasive tests such as FIB-4 or elastography.
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