APRI stands for AST-to-platelet ratio index. It combines an AST blood-test result, the upper reference limit for AST and the platelet count in a single numerical index. It is used within some non-invasive liver-disease assessment pathways, but the result is not itself a measured fibrosis stage.
This calculator exposes the arithmetic so that the units and normalisation step can be checked. It divides AST by its upper limit, divides again by the platelet count expressed in 10⁹/L and multiplies by one hundred. There is no age, ALT, elastography or biopsy input. The page therefore calculates APRI specifically; it cannot substitute for a different liver score or a complete evaluation.
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How to Use This Calculator
Enter AST level from the blood report in IU/L, using the result for the assessment being discussed.
Enter the AST upper limit using the same activity unit. Take the appropriate limit from the reporting laboratory or the assessment protocol specified by the clinician; do not substitute an ALT limit.
Enter platelet count in 10⁹/L. For a report giving 150 × 10⁹/L, the entry is 150. Check a count printed in another notation before typing the number.
Calculate and read the numerical index, rounded to two decimal places. Keep the AST, upper limit and platelet count with the result so that it can be reproduced.
Treat the accompanying category text cautiously. It is a simplified threshold label in this implementation, not a confirmed diagnosis or proof that significant disease is absent.
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Formula Used
APRI = [(AST/AST upper limit) / platelet count in 10⁹/L]×100. Both the upper limit and platelet denominator must be positive for the ordinary calculation.
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Variables Explained
Input
Meaning in this calculator
AST level
Aspartate aminotransferase activity reported in IU/L. It is the numerator before normalisation.
AST upper limit
Upper reference limit in the same unit as AST. The ratio AST/upper limit expresses AST as a multiple of that reference.
Platelet count
Platelets expressed as a numerical count in 10⁹/L. Entering a count per microlitre without adjusting its scale creates a major error.
APRI value
A calculated index, not a percentage of fibrosis and not a liver-enzyme concentration.
Category message
A threshold description returned by the page. Its certainty is more limited than labels such as “No significant fibrosis” might suggest.
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Worked Example
For AST of 60 IU/L, an AST upper limit of 40 IU/L and platelets of 150 × 10⁹/L, the first step is 60/40 = 1.5. Dividing by 150 gives 0.01. Multiplying by 100 gives APRI = 1.00. This is the value produced by entering 60, 40 and 150.
Now change platelets to 100 while leaving AST and its reference limit unchanged. APRI becomes (1.5/100) × 100 = 1.50. Alternatively, retaining platelets of 150 but changing the AST upper limit to 30 gives (60/30)/150 × 100 = 1.3333, displayed as 1.33. These examples show why both the platelet scale and the reference limit must be recorded. A different number can result from a changed denominator even when the AST measurement is identical.
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Why Use This Calculator?
WHO publishes the same AST-normalised platelet formula in its liver-disease guidance. Its clinical use is pathway-specific: one cutoff is not a universal diagnostic boundary across every liver condition and population. The calculator’s four displayed bands are below 0.5, 0.5 to below 1.5, 1.5 to below 2 and 2 or more. The controller chooses a band before rounding the displayed index, so a near-boundary result may look equal to a cutoff while retaining the adjacent label.
The University of Washington’s Hepatitis C Online calculator also emphasises that APRI alone cannot reliably exclude significant disease. Read this page’s stronger category wording as a simplified implementation label rather than a clinical conclusion. Changes in AST or platelet count need their own interpretation. The index contains no information about why either changed, and a calculator cannot determine whether results collected at different times belong in the same assessment.
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Common Mistakes
Do not enter ALT instead of AST. The enzymes are not interchangeable in the APRI expression.
Do not use zero for the AST upper limit or platelet count. Both are divisors.
Do not enter 150,000 when the report corresponds to 150 × 10⁹/L. The numeric platelet scale is part of the formula.
Do not interpret APRI 1 as one percent fibrosis or use a low value to conclude there is no clinically important liver disease.
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Frequently Asked Questions
Why is the AST upper limit needed?
The formula uses AST relative to a reference limit rather than the raw enzyme value alone. AST 60 with a limit of 40 is 1.5 times that limit; with a limit of 30 it is twice the limit. That difference flows directly into the final index.
Is 150,000 platelets per microlitre entered as 150?
Yes, that count corresponds to 150 × 10⁹/L, the scale requested here. Keep the unit conversion explicit rather than copying the printed digits alone. If the report uses a different notation or the units are unclear, confirm the numerical scale before calculating.
Does the result tell me a fibrosis stage?
No. The index can inform a defined assessment pathway, but it does not directly measure or assign a histological stage. The text next to the result is a simplified threshold message. Further interpretation depends on the condition being assessed and the wider evaluation.
Is APRI the same as FIB-4?
No. APRI uses AST, the AST upper limit and platelets. FIB-4 includes age and ALT as well as AST and platelets in a different expression. The values are not interchangeable, and entering a FIB-4 result into an APRI field does not convert one score into the other.
Why might a rounded result and label seem inconsistent?
This controller tests the unrounded index against its cutoffs and then rounds the number to two decimal places for display. A value just below 1.5 can appear as 1.50 while still carrying the lower category. Retain more precision when checking a boundary, and avoid treating that boundary as diagnostic certainty.
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