FIV testing Elisa vs. Western Blot

I just found a stray and after being unable to locate anyone to claim him, I took him to the vet before I let him near my four cats.  The stray tested postive for FIV using the Snap combo FIV/FELV test, which is a shame because I may have adopted him otherwise.  I seem to be finding some information saying that the snap test can produce false positives and have seen that it may only be around 70% accurate.  But I also see lots of recommendations on sites like Cornell to "ALWAYS"  follow up on a positive result with a Western Blot test.  Can anyone add anything here?  I'm confused as to why I can't find anything specific on the accuracy of the snap test and am also wondering how often a Western Blot contradicts a snap test for FIV.

Any insight is appreciated.

I would do the follow up test. From the research I did on this test when I had it performed on Porter, a negative is a negative with this test, but a positive is a maybe. Two tests using different testing methodologies is the only way to confirm the diagnosis of FIV. 

Good luck, I'm hoping for a false positive. 

Edited to Add: From what I have found, there seems to be a 32% rate of false positive on the snap test. The snap test is done via a color changing method. So the accuracy depends on the visual acuity of the person reading the results. Western Blot is an antibody test, which is much more easily discerned than a simple color change in a test tube. 

Hi, and welcome to TCS!

I'm going to answer your question in two parts. There will be a lot of links and information. But hopefully this will not just answer your question, but persuade you to go adopt that kitty, whether or not he's FIV+, because he poses almost no risk to your other cats. :heart2:

If your vet said otherwise, your vet is not knowledgeable about the disease, and should you - based on ACCURATE information - decide to adopt that boy who needs you, I suggest you call around to find an FIV literate vet. :rub: FIV is not FeLV, and many vets do not understand the risk differences, lumping them together. This could not be further from the truth. Many cats are being unnecessarily killed and not adopted because of the misunderstanding and myths surrounding FIV.

First: addressing the issue of casual transmission of FIV and FIV-related resources. This is THE best site on FIV. http://www.fivtherapy.com/index.htm There is no better collection of accurate, science-based information and referenced articles. The author of this site runs a yahoo group to provide information and support to FIV+ cat owners/rescuers. https://groups.yahoo.com/neo/groups/fiv-healthscience/info

This is the page dedicated to information on casual transmission of FIV: http://www.fivtherapy.com/fiv_casualtransmission.htm

Many in rescue will claim there is no risk to adopting an FIV+ kitty to your other cats. This isn't completely accurate. If your cats viciously fight, there is a big risk, because it is generally understood that FIV is passed via deep tissue bite wounds. If your cats are not sterilized, there is a risk, because it can be passed by mating. I very much doubt either situation is the case. There are also (rare) cases in the literature where FIV has been subsequently detected in a population of indoor-only cats that had an FIV+ kitty - but the information about what happened is lacking. They don't know that a bite didn't happen and the owner didn't notice. This is discussed in the link above.

If you've read through the Cornell literature on the subject of FIV, I'm sure you saw this:

The primary mode of transmission is through bite wounds. Casual, non-aggressive contact does not appear to be an efficient route of spreading FIV; as a result, cats in households with stable social structures where housemates do not fight are at little risk for acquiring FIV infections.

Click to expand...

(From: http://www.vet.cornell.edu/FHC/health_resources/brochure_fiv.cfm ).


I have now nine indoor-only pet cats in a small home (we live in an RV). We rescued an FIV+ cat in May 2010 (Chumley). He is best buddies with one of our other cats - constantly grooming him, cleaning his ears. When we had the boys in for their dentals a few months ago, we decided just to make sure Billy wasn't FIV+. Nope. Billy remains FIV negative.




As to how often the ELISA SNAP test reports false positives? There are studies indicating differing rates, and vets also report different rates.

Here is an article in the National Veterinary Laboratory Newsletter from 2009 where it references a study that found the false positives on the SNAP test were 20%. But their own recently published study found a rate of 32% false positives. http://www.natvetlab.com/PDF/NVLNewsLtr-Spring-2009-Vol.8 No.2.pdf This has a short synopsis of reasons for the errors.

And here is discussion of the testing, and why it can be inaccurate: http://www.fivtherapy.com/fiv_testing.htm

Personally, I wouldn't waste the money on a Western Blot. Either you decide you don't mind adopting and FIV+ kitty, or you do mind. But if you love that kitty enough to even be considering the Western Blot... I'd (obviously ) recommend adopting him, knowing he may need more care later in life. Of course - he may well not. Most FIV cats face the same illness disease as other cats do and end up dying of those issues.

We spent the money on a Western Blot with our Chumley (after he was already part of our home and family), just to be sure, just so we knew whether or not we needed to be more aggressive with treatment for him if he ever appeared to be sick. We don't like using antibiotics for every little thing, and we like to let our cats' immune systems do their work. We support them with a very healthy diet and certain supplements. So far, Chumley has remained very, very healthy. :cross:

Good luck with your decision.

I realize this is long  after the fact but I took in a stray and brought her to vet.was told she was FIV + and recommended I euthanize her.since she seemed fairly happy and healthy at the time and I had no other cats,I decided to take her home and do some research .I read ,like you did ,that accuracy was kind of sketchy and best to perform more sophisticated test.For more than $200,local vet took blood and sent to veterinary hospital about 2 hours away.Result was negative! that was nearly 6 years ago. So glad I didn't euthanize this kitty and it was irresponsible of vet to recommend without at least letting me know there was a more definitive test available.