The APRI Score Calculator computes the AST to Platelet Ratio Index from serum AST, upper limit of normal AST, and platelet count. A non-invasive, widely validated marker for hepatic fibrosis used in hepatitis C, hepatitis B, and NAFLD management to guide liver biopsy decisions.
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Does this hepatitis C patient need a liver biopsy? Before non-invasive fibrosis markers existed, the answer required sticking a needle into the liver — a procedure with real complication risk. The APRI score changed that. The calculator for APRI computes this validated non-invasive fibrosis marker from two routine blood tests, providing probability estimates for significant fibrosis and cirrhosis that help clinicians decide when liver biopsy is truly necessary.
APRI (AST to Platelet Ratio Index) was developed by Wai et al. (2003) and validated across multiple hepatitis C cohorts:
APRI = (AST / AST_ULN) × 100 / Platelet count (10⁹/L)
where AST is the patient's serum aspartate aminotransferase, AST_ULN is the upper limit of normal for AST in your laboratory's reference range (typically 40 U/L for men, 35 U/L for women), and platelet count is in 10⁹/L (= thousands per μL). Example: AST = 60 U/L, AST_ULN = 40 U/L, platelets = 120 × 10⁹/L; APRI = (60/40) × 100 / 120 = 1.50 × 100 / 120 = 1.25. The FIB-4 index calculator provides the complementary non-invasive fibrosis score that adds ALT and age.
WHO and EASL (European Association for the Study of the Liver) guidelines use the following APRI thresholds for hepatitis C:
Performance is highest in extreme values (very low or very high APRI) and least informative in the intermediate range, where additional testing is recommended. These thresholds apply primarily to hepatitis C — thresholds for NAFLD and hepatitis B may differ slightly.
The biological rationale for APRI is compelling. As hepatic fibrosis progresses to cirrhosis, portal hypertension develops — elevated portal venous pressure causes splenomegaly (enlarged spleen), which sequesters and destroys platelets (hypersplenism), reducing circulating platelet counts. Simultaneously, ongoing hepatocyte damage releases AST into the bloodstream. Advanced fibrosis and cirrhosis therefore produce the exact pattern APRI detects: rising AST (hepatocyte injury) plus falling platelets (portal hypertension). Neither AST nor platelet count alone is specific for fibrosis, but their ratio amplifies the fibrosis signal while partially canceling confounders. The MELD score calculator and liver disease calculators provide complementary hepatic function assessment tools.
APRI has moderate diagnostic accuracy (AUROC approximately 0.77–0.83 for significant fibrosis) — better than chance but insufficient to replace biopsy for individual treatment decisions in borderline cases. Confounders that raise AST independently of fibrosis (acute flares, coexisting conditions, medications, exercise) can elevate APRI without fibrosis progression. Thrombocytopenia from non-hepatic causes (immune, drug-induced, bone marrow) will also elevate APRI. In practice, APRI performs best when combined with FIB-4 and/or liver stiffness measurement by transient elastography (FibroScan), and when interpreted in the full clinical context of the patient's liver disease history.
APRI = [(AST / AST Upper Limit of Normal) / Platelet Count (x10^9/L)] x 100. For significant fibrosis: < 0.5 (low risk), > 1.5 (high risk). For cirrhosis: < 1.0 (low risk), > 2.0 (high risk). Values between cutoffs are indeterminate and require further evaluation.
APRI < 0.5 effectively rules out significant fibrosis with ~80% NPV. APRI > 1.5 suggests significant fibrosis warranting further investigation. APRI < 1.0 makes cirrhosis unlikely. APRI > 2.0 strongly suggests cirrhosis. Indeterminate values need elastography or biopsy.
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[(30/40)/250] x 100 = 0.30. Low fibrosis and cirrhosis risk.
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[(95/40)/85] x 100 = 2.79. High probability of both fibrosis and cirrhosis.
APRI is the AST to Platelet Ratio Index, a simple non-invasive score predicting liver fibrosis using only AST and platelet count. It was developed for hepatitis C but is applicable across liver diseases.
Different laboratories have different reference ranges for AST. Normalizing to the ULN creates a standardized ratio that can be compared across institutions regardless of the specific assay used.
FIB-4 generally outperforms APRI, particularly for NAFLD. However, APRI is simpler (only 2 variables) and is WHO-recommended for resource-limited settings. Both can be used complementarily.
WHO recommends APRI (with a cutoff of 2.0 for cirrhosis) as the preferred non-invasive test for fibrosis assessment in hepatitis B and C in settings where elastography is unavailable.
APRI can reduce need for biopsy in patients with clearly low (<0.5) or high (>1.5) scores. Patients in the indeterminate zone may still need biopsy or elastography for definitive staging.
APRI has moderate accuracy in NAFLD, with AUROC of approximately 0.70-0.75 for significant fibrosis. FIB-4 is preferred as the initial screening tool for NAFLD per AASLD/EASL guidelines.
Use your laboratory's reference range upper limit. Commonly 35-40 U/L for most laboratories. The specific value affects the score significantly, so accuracy matters.
Yes. APRI typically improves (decreases) with successful antiviral therapy as AST normalizes and platelet count may improve. However, APRI may not accurately reflect fibrosis regression in the short term.
Yes. Serial APRI measurements can track disease progression or improvement over time. Increasing APRI suggests fibrosis progression; decreasing APRI may indicate regression or treatment response.
Moderate accuracy compared to elastography, large indeterminate zone, unreliable during acute hepatitis flares, and lower performance in NAFLD than viral hepatitis. It should be one part of a comprehensive assessment.
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