APRI score: how to calculate the AST to platelet ratio index
The AST to platelet ratio index (APRI) is AST divided by the laboratory's AST upper limit of normal, divided by the platelet count in 10⁹/L, times 100. With an AST of 90 U/L, an upper limit of 45 U/L and platelets of 120 × 10⁹/L, APRI = ((90 ÷ 45) ÷ 120) × 100 = 1.67.
Not a medical device. Not a diagnosis or dosing recommendation.
What is APRI?
APRI is a simple index built from two routine blood tests: aspartate aminotransferase (AST) and the platelet count. It was designed to reflect liver fibrosis, the scarring that builds up in chronic liver disease. Its authors wrote that liver fibrosis has opposing effects on AST and on the platelet count, and they designed the index to amplify them by putting AST on top and platelets underneath.
It was introduced by Wai and colleagues in 2003 in patients with chronic hepatitis C. It's also called the AST/platelet ratio, the APRI score or Wai APRI.
This page explains the arithmetic. It doesn't give a cutoff, and it doesn't say what a value means for a patient.
The APRI formula
APRI = ((AST / AST_ULN) / platelet_count_10e9_L) * 100In words: divide AST by the lab's AST upper limit of normal, divide that by the platelet count in 10⁹/L, then multiply by 100. Magent's calculator reports APRI to 2 decimal places.
What each part means
| Term | Plain-English meaning | Unit | Accepted range |
|---|---|---|---|
ast_u_l | Aspartate aminotransferase | U/L | 1–10000 |
ast_uln_u_l | The lab's upper limit of normal for AST | U/L | 5–200, required |
platelets_10e9_l | Platelet count | 10⁹/L | 1–2000 |
apri | AST to platelet ratio index | index, no unit | 2 decimal places |
How the formula works
- Divide the AST by the AST upper limit of normal. This says how many times above (or below) the upper limit the AST is.
- Divide that number by the platelet count in 10⁹/L.
- Multiply by 100.
- Round to 2 decimal places.
AST and platelets push in opposite directions. Doubling AST doubles APRI. Halving the platelet count also doubles APRI.
Units
Two inputs deserve a check:
- The AST upper limit of normal is your lab's own value. It's printed with the result. Magent's calculator requires it and doesn't assume a default such as 40 U/L, because a different upper limit changes the answer.
- Platelets in 10⁹/L. That's the same number as thousands per microliter. Enter the count in thousands, as in the 120 of the Wai example, not the full per-microliter figure.
Worked examples
These use Magent's own expected values for the calculator.
Example 1: the Wai 2003 paper example (AST 90, upper limit 45, platelets 120)
- AST ÷ upper limit: 90 ÷ 45 = 2
- ÷ platelets: 2 ÷ 120 = 0.01666…
- × 100 = 1.666… → 1.67
Example 2: AST 40, upper limit 40, platelets 200
- 40 ÷ 40 = 1
- (1 ÷ 200) × 100 = 0.50
Example 3: AST 40, upper limit 40, platelets 100
- (1 ÷ 100) × 100 = 1.00
Example 4: AST 90, upper limit 40, platelets 80
- 90 ÷ 40 = 2.25
- (2.25 ÷ 80) × 100 = 2.8125 → 2.81
Example 5: AST 151, upper limit 100, platelets 100
- 151 ÷ 100 = 1.51
- (1.51 ÷ 100) × 100 = 1.51
Examples 2 and 3 have the same AST. Halving the platelet count from 200 to 100 doubles APRI from 0.50 to 1.00.
APRI reference table
These values are computed from the formula and rounded to 2 decimals, as the calculator reports them. Rows are AST as a multiple of the upper limit of normal; columns are platelets in 10⁹/L. They show how the index moves; they aren't targets, reference ranges or fibrosis stages.
| AST ÷ upper limit \ platelets (10⁹/L) | 80 | 100 | 120 | 150 | 200 | 250 |
|---|---|---|---|---|---|---|
| 1 | 1.25 | 1.00 | 0.83 | 0.67 | 0.50 | 0.40 |
| 2 | 2.50 | 2.00 | 1.67 | 1.33 | 1.00 | 0.80 |
| 3 | 3.75 | 3.00 | 2.50 | 2.00 | 1.50 | 1.20 |
| 4 | 5.00 | 4.00 | 3.33 | 2.67 | 2.00 | 1.60 |
Example 1 is the cell for 2 and 120.
History of APRI
The index comes from C. T. Wai, J. K. Greenson, R. J. Fontana and colleagues, A simple noninvasive index can predict both significant fibrosis and cirrhosis in patients with chronic hepatitis C, published in Hepatology in 2003 (volume 38, issue 2, pages 518–526).
The abstract describes consecutive, previously untreated patients with chronic hepatitis C who had a liver biopsy over 25 months. They were split into a training set of 192 patients and a second set of 78. The authors wrote that APRI was developed to amplify the opposing effects of liver fibrosis on AST and platelet count, and that a simple index using readily available laboratory results could reduce the need for staging biopsies.
When it's used
A 2012 meta-analysis describes APRI as a tool with limited expense and widespread availability, and a promising noninvasive alternative to liver biopsy for detecting fibrosis.[1] It notes that biopsy is limited by invasiveness, complications, sampling error and variable interpretation.[1]
Where it's used
APRI has been studied beyond hepatitis C. The 2012 meta-analysis pooled studies in chronic hepatitis B.[1] It is built from two inexpensive laboratory tests that are performed routinely.[1]
Who uses it
Published work frames APRI as one of several noninvasive measures that researchers and clinicians have examined for grading liver fibrosis.[1]
Limits of the calculation
- It's a ratio of laboratory values. It doesn't stage fibrosis, and it isn't a biopsy.
- The calculator also returns the Wai 2003 published bands. This page doesn't show them.
- It was derived in chronic hepatitis C. Magent doesn't re-derive anything for other liver diseases.
- It isn't FIB-4 and it isn't the NAFLD fibrosis score.
- The AST upper limit of normal is required input. The calculator doesn't assume 40 U/L.
- Inputs outside the accepted ranges are rejected, not estimated.
Clinical decisions belong to a qualified clinician who knows the patient.
How it differs from related calculators
Glossary
- AST: aspartate aminotransferase, a liver enzyme measured in U/L.
- Upper limit of normal (ULN): the top of a lab's reference interval for a test, printed with the result.
- Platelet count: platelets per liter of blood, reported as 10⁹/L.
- Fibrosis: scarring of liver tissue.
- Cirrhosis: advanced, widespread fibrosis.
Common mistakes
- Leaving out the upper limit of normal. APRI divides AST by the lab's own upper limit, not by a fixed number.
- Entering platelets per microliter. Enter the count in thousands per microliter (10⁹/L), not the full per-microliter figure.
- Forgetting the × 100. Without it, the Wai example comes out a hundred times too small instead of 1.67.
- Mixing APRI with FIB-4. They use different inputs and give different numbers.
Source
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. DOI: 10.1053/jhep.2003.50346
Further reading
- Jin W, Lin Z, Xin Y, Jiang X, Dong Q, Xuan S. Diagnostic accuracy of the aspartate aminotransferase-to-platelet ratio index for the prediction of hepatitis B-related fibrosis: a leading meta-analysis. BMC Gastroenterol. 2012;12:14.
Frequently asked questions
How do you calculate APRI?
Divide AST by the lab's AST upper limit of normal, divide by the platelet count in 10⁹/L, and multiply by 100. For 90, 45 and 120, that's (2 ÷ 120) × 100 = 1.67.
What APRI score indicates cirrhosis?
This page doesn't give a cutoff. It calculates the index; what a value means depends on the clinical context.
What AST upper limit should I use?
The upper limit printed on the patient's own lab report. The calculator requires it.
Is APRI the same as FIB-4?
No. FIB-4 adds age and ALT; APRI uses AST, its upper limit and platelets.
What units are platelets in?
10⁹/L, which is the same number as thousands per microliter. A count reported per microliter is entered in thousands.
Who developed APRI?
C. T. Wai and colleagues, in a 2003 paper in Hepatology.