APRI (AST to Platelet Ratio Index) Calculator

Understanding the APRI calculator — concept and principle

The AST to Platelet Ratio Index (APRI) is a simple blood-test score for estimating liver fibrosis and cirrhosis, developed in chronic hepatitis C. Because it needs only AST and a platelet count, it is widely used where elastography or biopsy are not readily available, and the World Health Organization has recommended it for assessing fibrosis in hepatitis B and C programmes.

A high AST and a low platelet count both push APRI up; the default 0.83 sits between 0.5 and 1.

How it is calculated

APRI = (AST ÷ AST upper limit of normal) ÷ platelets × 100

AST in U/L, divided by your laboratory's upper limit of normal (ULN), often around 40 U/L.
Platelets in × 10⁹/L.
The factor 100 simply moves the result into a convenient range.

Following WHO 2014, the calculator labels values below 0.5 as fibrosis less likely, 1–2 as cirrhosis possible, above 2 as cirrhosis likely, and 0.5–1 as intermediate.

The principle

As fibrosis progresses, AST tends to rise and platelets tend to fall: scarring raises pressure in the portal vein, the spleen enlarges and traps platelets, and the liver produces less thrombopoietin. Dividing a rising number by a falling one amplifies the signal. Expressing AST as a multiple of the ULN makes results comparable between laboratories with different reference ranges.

Worked example

AST ÷ ULN = 60 ÷ 40 = 1.5.
1.5 ÷ 180 = 0.008333.
× 100 = 0.833, rounded to 0.83.
0.83 is between 0.5 and 1, so the result is intermediate.

Each point is the calculator re-run with only “AST upper limit of normal” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

AST upper limit of normal	APRI
10	3.33
20	1.67
30	1.11
40	0.83
50	0.67
60	0.56
80	0.42

How to read the result

Where does your result fall?

In the original study, APRI of 0.5 or less made significant fibrosis unlikely and above 1.5 made it likely; for cirrhosis, 1.0 or less made it unlikely and above 2.0 made it likely. WHO guidance uses above 2 to identify adults with cirrhosis, and above 1 as the more sensitive threshold. Many results fall in the intermediate zone, where a second method such as transient elastography gives a clearer picture.

Common mistakes

Entering AST without dividing by the ULN, or using a ULN from a different laboratory.
Using platelets in other units such as /µL without converting.
Calculating during an acute flare, when AST is temporarily very high.

Keep in mind: APRI is not a diagnosis. Its accuracy is moderate, and conditions such as low platelets from other causes, muscle injury or alcohol can distort it. A clinician should interpret it alongside other tests.

Frequently asked questions

What is a normal APRI score?

There is no single normal value, but results below 0.5 are generally reassuring.

Is APRI or FIB-4 better?

They perform similarly; FIB-4 adds age and ALT and is preferred in fatty liver disease guidance.

Can APRI be used for fatty liver disease?

It can be calculated, but it was developed in hepatitis C and FIB-4 is usually recommended instead.

Sources: Wai CT et al. A simple noninvasive index can predict both significant fibrosis and cirrhosis in patients with chronic hepatitis C. Hepatology 2003. · World Health Organization. Guidelines for the screening, care and treatment of persons with hepatitis C infection, 2014.

Checked 8 Nov 2026. Wai et al., Hepatology 2003; WHO 2014.

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