Kidney infection vs. Kidney failure

Question
Hello,

I took my three-year-old cat to the vet over the weekend to get the results of a blood test taken on Friday (she hadn't been eating, had been peeing on the carpet, and had lost a substantial amount of weight).  The blood test results showed that her BUN was about 220, and her creatine levels were really high, too.  She also had an elevated white blood cell count.  For the last three days I've been giving her antibiotics and 150 cc of sub-q fluids every day, and she seems to be doing far, far better (is actually eating, hopping around, not peeing on the carpet, etc.).

My question is this:  is there any chance she could just have a kidney infection instead of having failing kidneys?  How does a vet tell the difference?  

Thank you!

Answer
What did he tell you it was? Her kidney levels are pretty high and while an infection can raise them- usually not that high.

I think it's a matter of how she reacts to treatment more than anything else. The thing is, when a cat starts to show those kinds of levels in their blood work, about 80% of kidney failure has been compromised.

There are many types of kidney failure. One thing they can get is called prerenal azotemia. This happens when there is a disruption of the urine such as when a cat gets blocked.
Here is what the Merck manual says about azotemia:
Renal azotemia refers to a reduction in glomerular filtration rate (GFR) of ~75% during acute or chronic primary renal (or intrarenal) diseases. Postrenal azotemia develops when the integrity of the urinary tract is disrupted (eg, bladder rupture) or urine outflow is obstructed (eg, urethral or bilateral ureteral obstruction). Once adequate urine flow is restored, postrenal azotemia will resolve.

Notice there is a glomerular filtration rate of aprox 75%. So that would tell the vet that it could be azotemia instead of acute renal failure.
Azotemia is a multi-level problem in itself and it's too complicated to go into here.
If she responds to this treatment and you keep her on a lower protein diet for life (K/D is the best), then she could be fine for many years.

That is about all I can tell you at this point. Unless the vet found other anomalies such as polycystic kidneys, etc, only time will tell how she will be. She sounds like she is bouncing back and cats in acute renal failure that are really sick don't bounce back like that.

Good luck and I hope she makes a complete recovery. Let me know how she is in a while.