Perhaps one of the hardest things to do as an owner of a cat with renal failure is not to get too invested in the Numbers (Blood Urea Nitrogen, Creatinine, Phosphorous). The Numbers are an indicator of renal function--i.e., lower is better, because lower means that the kidneys are taking the stuff out--and, as such, it is often recommended that they be tested frequently. The Tweeb has an appointment with the much-dreaded vet about 3 times a year (evidenced by the pee stain on our couch), but depending on the severity of the case it can be as often as once a month.
But going strictly by the Numbers ignores the cat. The fact that the BUN and Creatinine have gone down slightly from the last visit sounds like a cause for relief, if not celebration. But if the cat is so miserable from the change in food and starts wasting away because it won't eat--well, that could also contribute to the decrease in BUN and creatinine, and it's probably not nearly so worthwhile.
I won't presume to make an assertion as to what's worse: starve the cat to death, or let it eat itself to death. Every cat is different, and every owner likewise. The point is that there is still a lot we don't know about cats, and even more we don't know about renal failure, and to treat by the Numbers alone is to ignore the overall status of the cat: is it still reasonably healthy? Does she still play? Has her personality changed? The gestalt often tells a more complete tale of how the therapy is working than just the Numbers.
We've recently started the Tweeb on a prescription diet (Science Diet), as her Numbers have been elevated for two tests in a row. Fortunately, she seems to love the stuff more than life itself (as does Shadow, who most emphatically does not have renal failure) and it seems to agree with her, though her coat is somewhat more scruffy than it had been. We've agreed to take her in about four months later to see how she's doing.
Four months is a long time. I've had the Tweeb for two years now--that makes almost two and a half years as a CRF kitty for her. They don't call it "chronic" for nothing, and that's the thing. It may seem like a hopeless fight--after all, it starts badly and can only get worse--but keep in mind that if it is indeed the chronic, idiopathic kind of renal failure, proper care can keep a cat going for years. The moment of diagnosis is not the moment to consider euthanasia, but a moment to seriously re-evaluate your commitment to your cat--the whole Cat.