Join this FB group ASAP and read as much info on their site as possible. I'd encourage you to avoid RX foods and read about all the success people with IBD cats have had with raw or homecooked.
All fecal tests including the PCR came back negative. The vet simply stated that Ruby has severe IBD. We are continuing the probiotic and Prednisolone and starting Metronidazole and suggested the prescription diet.
https://www.facebook.com/groups/RawFedIBD/
http://www.rawfeedingforibdcats.org/
http://www.foodfurlife.com/