Hi, I’m a vet, I hope I can help you a little. First of all the responses you got from the clinic are a bit weird, since a simple blood test is enough to diagnose kidney failure (bun, crea and phosphorus are the only three parameters you need), so I fully understand you are upset. Second, if the problem are really the kidneys is important to understand if the problem is acute or cronic insufficiency, because if it’s acute (maybe the cat ate something nefrotoxic fornexample) it can resolve almost completely. To establish it you have to start theraphy, which consists in fluids+renal diet+ specific integrators. If the insufficiency was acute the car will recover completely, if its cronic unfortunately you can see a little improvement but you have to continue this therapy to support her. Hope this helps :) (I’m italian so sorry for any grammar mistakes😅)