Interstitial Cystitis- This is a rare bladder disease humans get but it has been found that cats can also get it, human and cat bladders are quite similar in fact they are doing studies on cat's with Interstitial Cystitis to try and help humans.
I have Interstitial Cystitis this is why I know of this bladder disease it's very similar to a UTI and bladder cancer only that it does not show up has a UTI, and antibiotics will not help, And there is no blood in the urine. This is a life long disease also Symptoms are urinating a lot (I personally urinate about 40-50 times in 24 hours) and bladder pain.
Symptoms for cats that I know of are peeing a lot and/or not peeing where they know they are supposed to.
So if you do think you cat has a UTI but tests are negative you might want to talk to your vet about Interstitial Cystitis remember it is rare and it's hard enough for me to find doctors who know about it so finding a vet that knows about it might be just has hard. Now I don't know what treaments that vet will have for your cat but the only thing you can do for this disease is finding medications that help pain and holding urine.
I hope this helps some one, so they can help there cat.
