her kidney function BUN was not surprisingly for an old lady creeping up to the higher side of normal ... Wanting the best for my girl, I asked if it would be good to put her on a diet which would be sympathetic to her kidneys and help her to not develop any problems. He certainly didn't 'sell' it to me but did say we could try Hills Renal Diet.
Please forgive any abrupt tone that may come across in this response. My mother had a pretty bad stroke a few days ago, so I'm in hyper-logical mode right now trying to cope with all that's going on with her right now. So please don't take any offense if my tone seems impersonal. I just want to give you as much useful information as possible as succinctly as possible.
First, please get copies of all of your cat's lab results ASAP from your vet. You'll need them in order to make sense of the information I'll link for you, and we'll need to see them posted here in order to give you the best possible guidance.
While BUN is one of the blood values often associated with renal function, it can be elevated for non-renal reasons, as well. The more important blood value is creatinine, because it is kidney-specific. If the creat is elevated, there is something going on with your girl's kidneys.
Renal foods should not be fed to cats who are not in renal failure, and many people now believe that they are counter-productive any earlier than late-stage renal failure. The reduced protein in renal diets can seriously weaken a cat. This is why many people now believe that these foods should not be fed until the benefit outweighs the potential harm - namely, in late-stage renal failure. In addition to the issues related to protein deficiency, renal diets, as you've sadly discovered, are unpalatable to many cats. Cats will starve themselves to death rather than eat undesirable foods, and your girl was apparently on her way to doing so. It's a good thing that you caught the problem in time.
The rule of thumb with cats in renal failure is to offer them whatever is the optimal diet for their stage of disease, and if they won't eat that, then offer them whatever they WILL eat. The bottom line is that they MUST eat, regardless of what you have to feed them in order to get them to do so. You have done absolutely the right thing by switching her back to her regular food, though I also recommend that you slowly try mixing in a bit of higher quality foods like canned EVO or other premium canned foods to upgrade the quality of the protein sources in her diet.
she'd lost 1.5 kg since January and her glucose and calcium were very high. The vet told us she has probably got lymphoma in her kidneys and is diabetic.
Cats can spike very high blood glucose readings when stressed or in pain. Eating a meal within 8 hrs of the blood draw can also elevate glucose to some degree. A much more reliable way to test for diabetes is to request a fructosamine test. This is a blood test that averages the blood glucose over, I believe, several weeks, so it is not subject to those high-stress spikes.
Elevated calcium may be a much greater concern, though if her renal values were also elevated at that time, serum calcium is not necessarily an accurate test. If she still has elevated serum calcium now that she's feeling better, I strongly recommend that you have your vet send a new blood sample to Michigan State University's lab for an iCa and PTH test. MSU is generally regarded as the most reliable and one of the only labs capable of performing these particular tests. iCa tests ionized calcium, which is the only calcium that counts in a cat with renal disease. PTH tests for hyperparathyroidism, which is one potential cause of elevated calcium. The following link will provide you with MSU's information page on these tests:
Diagnostic Center for Population and Animal Health
Why would she be 'getting better' if she is actually that sick
Cats in renal failure can act perfectly normal, esp. if they've been starving and suddenly start eating again. So, I recommend you get her back to your vet for a full blood chemistry now that she's feeling and acting better and see what her status is now. If she is still having renal issues, or if her calcium or glucose are still elevated, further testing of fructosamine, iCa & PTH, and a urinalysis would be in order to clarify her condition.
It doesn't sound to me like a death sentence is justified without a lot more diagnostic information on your girl.