APRI calculator

The formula

APRI = (AST ÷ AST upper limit of normal) × 100 ÷ platelet count, with platelets in 10⁹/L (Wai 2003) (WHO 2014 (hepatitis C)). The upper limit of normal is the one from the lab that ran your AST test (WHO 2015 (hepatitis B)). WHO gives this example: an AST of 82 U/L with a lab upper limit of 40 U/L and a platelet count of 90 gives (82 ÷ 40) × 100 ÷ 90 = 2.28, which is over 2 and consistent with cirrhosis (WHO 2015 (hepatitis B)).

Scarring in the liver tends to push AST up and the platelet count down, so the ratio amplifies both changes at once (Wai 2003).

The cutoffs: 0.5, 1.0, 1.5 and 2.0

WHO uses two cutoffs for each question. The low one is set to rule scarring out (few false negatives) and the high one to confirm it (few false positives), so results between them stay uncertain and need another test or a later retest (WHO 2014 (hepatitis C)).

WHO's 2015 hepatitis B guideline recommends APRI over 2 as the sign of cirrhosis in adults in resource-limited settings (WHO 2015 (hepatitis B)). Its 2024 update set new thresholds for hepatitis B: an APRI over 0.5 as evidence of significant scarring and over 1.0 (or clinical signs) for cirrhosis, in adults (WHO 2024 (hepatitis B)).

How accurate is it?

In the study that created it, APRI separated people with and without significant scarring with an area under the curve of 0.80, and cirrhosis with 0.89; in the second group used to test it, the figures were 0.88 and 0.94 (Wai 2003). WHO's review for hepatitis C found that the low cutoff of 0.5 picked up 82% of people with significant scarring, while the high cutoff of 1.5 caught only 39% of them but gave a false alarm to just 8% of people without it (WHO 2014 (hepatitis C)). For cirrhosis, the cutoffs of 1.0 and 2.0 picked up 77% and 48% (WHO 2014 (hepatitis C)).

In hepatitis B the test missed more: the cirrhosis cutoffs picked up 65% (low) and 35% (high) of people with cirrhosis (WHO 2015 (hepatitis B)). WHO warns that many cases of cirrhosis will be missed with these tests alone, and that heavy drinking, low platelets from conditions such as HIV or malaria, and some medicines or herbal remedies can make APRI falsely high (WHO 2015 (hepatitis B)).

APRI and fatty liver disease

APRI was built and tested in hepatitis C (Wai 2003). For people with fatty liver disease, the American Association for the Study of Liver Diseases recommends FIB-4 as the first test for advanced scarring (AASLD 2023). If you have AST, ALT and platelets, use the FIB-4 calculator, and for a second look that adds BMI, blood sugar and albumin, see the NAFLD fibrosis score calculator.

Who should think about this

APRI only means something if you know why your liver might be scarred. If you have never been tested for hepatitis C or hepatitis B, start with the hepatitis C screening check and the hepatitis B screening check. If you already have hepatitis C or B, ask the clinician who looks after it how your APRI fits with your other results, since a value above a high cutoff is a reason for a closer look, not a diagnosis.

Where do I find the AST upper limit?

On your lab report, next to your AST result, as the top of the reference range. WHO's formula uses the upper limit of normal of the lab where the test was done (WHO 2015 (hepatitis B)), so use the one printed on your own report rather than a number from another lab.