The AST-to-Platelet Ratio Index (APRI): A Good Test of Fibrosis
(Note: I composed this in Word, so if it doesn't format properly here, my apologies.)
Hi! friends, Medhelpers, and fellow sufferers from hepatitis and the medical profession. In a previous thread, concerned with the reliability of fibrosis tests like biopsy, Fibroscan, Fibrotest, FibroTC, etc., I mentioned a simple fibrosis index I recently discovered called the AST-to-Platelet Ratio Index, or APRI, that can be calculated from blood tests, and which has been shown to have a high level of reliability. A number of Medhelpers suggested I post the info about this fibrosis index as a separate thread, so here it is.
The only two data you need are the blood levels of aspartate aminotransferase (AST or SGOT), one of the liver enzymes measured in a Hepatogram and given as IU/L, and a platelet count, given usually as platelets/cubic mililiter (/mm3). You also need to know the Upper Limit of the Normal range (ULN) of AST, which my lab gives as 42 IU/L. (Some labs give 40 IU/L as the Upper Limit of Normal, but the difference in the final result is small.)
When you've got these numbers, you divide your AST by the ULN of AST, divide this result by the platelet count (with the last three zeros chopped off), and multiply by 100. As a formula it's (AST/ULN)/platelets x 100.
Here's an example of how it works, for an AST of 63 and a platelet count of 137,000:
Now, what does an APRI score of 1.09 tell me? Well, APRI comes with two cut-offs: a lower one, 0.5, and a higher one, 1.5. If the APRI score is less than or equal to 0.5, you have no fibrosis or just a little. If your APRI score is 1.5 or above, you probably have cirrhosis. APRI scores between 0.5 and 1.5 are related to progressive fibrosis stages, like Metavir F1-to-F4.
That's all there is to it. It's simple, has a good level of reliability compared to other fibrosis tests, and you can calculate it yourself from your blood tests. However, like all known biomarker fibrosis tests, you only get out what you put in, and the levels of AST and platelets can vary for reasons other than your fibrosis stage. So the best way of using APRI is as a progression over time. You can do that by having staged blood tests, calculating the APRI from each, and making a graph or chart of the results. This should give you a good idea of how fast you are progressing, as well as your fibrosis stage at any point in time.
I've had eight blood tests since 2005, most of them in the past year and a half after I was diagnosed with chronic hep C. I calculated my APRI scores from all these blood tests and got this sequence: 0.50 (04/2005), 0.42 (04/2008), 0.72 (05/2008), 0.67 (07/2008), 1.01 (08/2008), 0.93 (12/2008), 1.09 (04/2009), 1.38 (09/2009).
What does this tell me? First, that I had little fibrosis in 2005, and that my liver remained only partially fibrotic until early in 2008. (The apparently out-of-sequence 0.42 score for April, 2008, is just a result of the vagaries of this test. It is not perfectly accurate for each score.) Unfortunately I wasn't diagnosed until 2008, so no blood tests were done in 2006 and 2007 (at least, none with both AST and platelets measured). In 2008, with the renewal of complete blood tests, my fibrosis had increased; and my liver is now so fibrotic that it is partially cirrhotic.
These indications from my APRI are reflected in the other tests done. A biopsy in mid-2008 showed Metavir F1/F2 (two pathologists studied the specimen, one giving it an F1, the other an F2), which correlates with an APRI between 0.5 and 1. A recent Fibroscan gave me F3/F4, which correlates with my most recent APRI of 1.38. So everything seems to hold together.
If you're interested in reading up on the APRI index, here are a couple of references:
Snyder N, Gajula L, Xiao SY, Grady J, Luxon B, Lau DT, Soloway R, Petersen J. APRI: an easy and validated predictor of hepatic fibrosis in chronic hepatitis C. J Clin Gastroenterol. 2006 Jul;40(6):535-42.
Ngo Van Huy, et al. Predictive Factors of Advanced Fibrosis on Liver Biopsy in Hepatitis C in Vietnam. (Presented at the 2009 AASLD)
i got 0.6 APRI over a few bloods - i had biopsy in 2006 and was diagnosed with Fibrosis 2/6 , Inflammation 7/18
Stage2Grade7 i think but i dunno which scoring method was used - im in UK , aso i was heavy drinker just at that time - i gave up immediatley after biopsy and started milk thistle - i had Fibroscan in 2008 which came back 3.3 and normal ultrasound...
does this reflect my APRI score do you think? F2 in 2006 and now 0.6 APRI and 3.3 on Fibroscan
The problem is that in many hepatitis C patients AST/ALT fluctuate widely. For example, my husband had AST 15 in July, but in September it was 100. Platelets are 110. If we consider the first set of labs, he has no fibrosis at all, if we take second set-he has cirrhosis. Maybe this would be better for people whose enzymes are more stable.
An APRI score of 0.6 is pretty low. My biopsy in Feb. 2008 was F1/F2, and my APRI for then was only 0.5. So maybe an APRI of 0.6 could correlate with a biopsy of F2. But don't forget that none of these tests is that accurate. Both biopsy and APRI, as well as Fibroscan and all the others, are really just aids. And they are most useful as time studies, comparing one time point with the next (in the same test).
When you say Fibroscan of 3.3, do you mean the kPa score of the scan? If so, that's very low.
That's why I suggest the APRI works better over time. Not all the blood test results are accurately reflecting the state of the liver. Fibrosis is cumulative, whereas AST is immediate and variable. Like you say, AST can vary widely: it depends on how many hepatocytes are dumping their AST into the blood at any given point in time. But over the long run, the AST-to-platelet ratio has shown to be a good index of fibrosis progression. It was not developped as a short-term test. It's utility is over the mid and long term. Mine happens to work over a relatively short span of time because, like you say, my liver enzymes are increasing fairly linearly, which is not always the case.
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