Opinions: FIV false negatives, false positives

Hello, I have two cats. One just tested positive for FIV, and the other just tested negative. Both cats have another appointment for a second opinion, but I am concerned in the meantime whether one or the other has a false positive or false negative because the narrative does not make sense as the indoor cat tested positive and outdoor cat negative. More info below. Any insight is greatly appreciated! Thank you! Is a false pos or a false neg more common? Cat #1 - 4 years old Cat #2 - 11 years old Cat #1 that does not go outside, and has not fought with any other cats (besides other house cat) tests positive Cat #2 that does go outside and fights outdoor cats tests negative Both cats have “chronic” issues Positive cat - respiratory illness (presents as asthma / coughing fits 2-6x daily) — currently presumed respiratory infection, and being treated with antibiotics Negative cat - skin issues (itching, hair loss), frequent vomiting — previously presumed food or environmental allergy

Hello - and what an interesting question.  I'm going to pick up on the almost irrelevant point at the bottom first though, and explain that recurrent vomiting is not commonly caused by allergy in cats and that medical diseases such as pancreatitis and hyperthyroidism as well as physical diseases such as foreign bodies - usually need to be ruled out.

Hello, and thank you for your response! Cat #2 has had rather frequent vomiting for about 3 years. I looked at some old paperwork from our vet and his latest testing included bloodwork, urinalysis and abdominal x-rays on 2/11/2021 which “showed no obvious cause for his chronic vomiting. Gastroenteritis is suspected.” It was then suggested he be put on royal canin hydrolyzed protein diet which seemed to help initially, but he still vomits anywhere from 0-4x / week currently. Maybe it is possible the labs did not check for hyperthyroidism or pancreatitis since I’m not sure exactly what they were testing/looking for. Once cat #1 tested positive for FIV, we suspected that maybe some of cat #2’s issues were caused by FIV rather than allergies as suspected. But then his FIV test came back negative. Anyway, apologies for the lengthy paragraph. I am just stumped and wanted to give all the info. Again, thank you!

Hello!   Thanks for that.  Yes - there are specific pancreatitis tests and they are often not found on routine blood profiles, so this is something to ask your vet about.  Pancreatitis is a very common cause of vomiting in adult cats, but there are others.  If vomiting is the main sign, then this could be worth investigating independently.  FIV is a virus that is transmitted between cats (very close contact required).  It can be similar to the HIV virus, in that it can be present without doing any damage for years and sometimes maybe a lifetime.  It is quite possible for cats to be FIV +ve without any apparent symptoms of illness and FIV -ve with symptoms of illness, caused by different things.

In short, my obvious next step would not be to start questioning the validity of the FIV test because those answers could be perfectly reliable and you still see the signs you are seeing.  However, it is always good to be aware of the false +ves / -ves / reliability of a test anyway, and your vet (who knows what kind of test they did) should be able to discuss them with you.  

Because a cat tests negative for aids, they could still have something else wrong in their body causing them to vomit.  We would recommend that you talk this over with your vet and they do a work-up to see if they can help with this problem.  Vomiting can be caused by hair-balls, pancreatitis, kidney diease, liver disease, etc but it isn't 'normal' for cats.  Meanwhile, regarding the cat that tested positive, good questions for your vet include:  What does this mean for my cat?  What implications does this have for the two cats living together?  What should I look out for in the future?    Hopefully this will prove a useful conversation.