APRI Calculator (AST to Platelet Ratio Index)
The APRI (AST to Platelet Ratio Index) is a non-invasive blood-test score used to estimate the likelihood of significant liver fibrosis and cirrhosis - particularly in people with hepatitis C. Enter your AST level, your lab's upper limit of normal for AST, and your platelet count to get your APRI score along with its clinical interpretation.
Formula
Worked example
A patient has AST = 60 IU/L, AST ULN = 40 IU/L, platelet count = 120 x10⁹/L. AST/ULN = 60/40 = 1.5. APRI = (1.5/120) x 100 = 1.25. A score of 1.25 is above the 1.0 cirrhosis cutoff, suggesting significant liver fibrosis or cirrhosis.
What is the APRI score?
The AST to Platelet Ratio Index (APRI) is a simple, non-invasive formula developed to estimate the degree of liver fibrosis and cirrhosis using two routine blood tests: aspartate aminotransferase (AST) and platelet count. It was originally developed by Wai and colleagues in 2003 and validated extensively in hepatitis C virus (HCV) populations as an alternative to liver biopsy. Because both AST and platelet count are part of a standard metabolic panel and complete blood count, the APRI can be calculated from lab work a patient likely already has, making it a cost-effective first-pass screening tool.
How is APRI calculated?
The formula is: APRI = (AST / AST_ULN) / Platelet_Count x 100, where AST is measured in IU/L, AST_ULN is the upper limit of normal for AST at your laboratory (most labs use 40 IU/L), and platelet count is in units of 10⁹/L. A raised AST pushes the score higher, reflecting hepatocellular injury. A low platelet count also pushes the score higher, reflecting portal hypertension and splenic sequestration that often accompany advanced fibrosis. These two mechanisms make APRI physiologically sensible as a fibrosis proxy.
How to interpret your APRI score
Based on a meta-analysis of 40 studies (Shaheen and Myers, 2007), the widely used cutoffs are: below 0.5 - has strong negative predictive value for ruling out cirrhosis; above 0.7 - suggests significant fibrosis (METAVIR stage F2 or higher), with sensitivity 77% and specificity 72%; above 1.0 - suggests cirrhosis with sensitivity 76% and specificity 72%; above 1.5 - has strong positive predictive value for cirrhosis; above 2.0 - reaches specificity of 91% for cirrhosis. Values between 0.5 and 1.0 are considered indeterminate and require additional evaluation. The APRI is most validated in HCV populations and may perform differently in non-alcoholic fatty liver disease (NAFLD) or other liver conditions.
Limitations and when to use other tests
The APRI is a screening index, not a diagnostic test. It has only moderate accuracy overall, and values in the mid-range (0.5 to 1.5) are difficult to interpret in isolation. The score can be elevated by conditions that raise AST without liver fibrosis, such as hemolysis, myopathy, or thyroid disease. Low platelets from causes other than portal hypertension (such as immune thrombocytopenia) can also falsely elevate the score. For most clinical decisions, APRI is best combined with other non-invasive markers such as FIB-4 Index, FibroTest, or transient elastography (FibroScan). Liver biopsy remains the reference standard when non-invasive tests give discordant or indeterminate results.
APRI score interpretation cutoffs
| APRI score | Clinical finding | Sensitivity | Specificity |
|---|---|---|---|
| < 0.5 | Cirrhosis ruled out (high NPV) | - | High NPV |
| > 0.7 | Significant fibrosis (F2+) | 77% | 72% |
| > 1.0 | Cirrhosis | 76% | 72% |
| > 1.5 | Cirrhosis confirmed (high PPV) | - | High PPV |
| > 2.0 | Cirrhosis (high specificity) | 46% | 91% |
Meta-analysis derived cutoffs from Shaheen and Myers (2007) across 40 studies in hepatitis C populations.
Frequently asked questions
What does a high APRI score mean?
A high APRI score suggests a greater likelihood of significant liver fibrosis or cirrhosis. An APRI above 0.7 is associated with significant fibrosis (sensitivity 77%, specificity 72%), and a score above 1.0 is associated with cirrhosis (sensitivity 76%, specificity 72%). Scores above 1.5 have strong positive predictive value for cirrhosis. A high score does not confirm fibrosis on its own - a hepatologist should review results alongside imaging and other clinical data.
What is a normal APRI score?
There is no single "normal" APRI the way a reference range works for individual lab tests. Clinically, an APRI below 0.5 has good negative predictive value for ruling out significant fibrosis and cirrhosis. Many healthy adults with normal liver function would fall well below 0.5. Values between 0.5 and 1.0 are considered indeterminate and require further evaluation.
What upper limit of normal (ULN) should I use for AST?
Most published APRI studies and the original Wai 2003 validation used 40 IU/L as the AST ULN. This is also the most common lab reference value. However, your laboratory may report a slightly different ULN depending on the assay method and the reference population. Always check your own lab report and use that value for the most accurate APRI calculation.
Can APRI replace a liver biopsy?
APRI can help avoid biopsy in patients with scores clearly below 0.5 (low risk) or clearly above 2.0 (high risk with 91% specificity for cirrhosis). In the large grey zone between those extremes, APRI alone is not sufficient. Guidelines generally recommend combining APRI with at least one other non-invasive marker - such as FIB-4 or transient elastography - before considering biopsy avoidance. Biopsy remains the gold standard for staging fibrosis.
Is APRI validated for conditions other than hepatitis C?
The strongest evidence for APRI is in hepatitis C (HCV) populations. Studies in other conditions - including non-alcoholic fatty liver disease (NAFLD), hepatitis B, and alcoholic liver disease - show more variable accuracy. The WHO has endorsed APRI as a first-line non-invasive test in resource-limited settings for HCV. For other liver conditions, FIB-4 or condition-specific models may be more appropriate.
What other tests are used alongside APRI to assess liver fibrosis?
The most commonly paired tests are FIB-4 Index (which uses age, ALT, AST, and platelets), FibroTest (a proprietary serum panel), and transient elastography (FibroScan, which physically measures liver stiffness). Using two concordant non-invasive tests substantially improves confidence in the fibrosis stage estimate. If tests disagree, liver biopsy is usually the next step.
Sources
- Wai CT et al. A simple noninvasive index can predict both significant fibrosis and cirrhosis in patients with chronic hepatitis C. Hepatology 2003.
- Shaheen AA, Myers RP. Diagnostic accuracy of the aspartate aminotransferase-to-platelet ratio index for the prediction of hepatitis C-related fibrosis. Am J Gastroenterol 2007.
- University of Washington Hepatitis C Online - AST to Platelet Ratio Index (APRI) Calculator