Our experience here has just been with bladder infection and crystals in
the urine, but you might want to read this and see if there are more
tests that could help pinpoint it:

http://www.winnfelinehealth.org/health/urinary-tract.html

Hope you can fine out what's bothering her bladder!

Rhonda

You were right.  Inflammation in the bladder and/or urethra produces a
nervous sensation that's the same as the sensation that's induced by a full
bladder. The nervous impulses that control the urge to pee are constantly
stimulated so that the urge to pee is constant whether the bladder is full
or empty.  Even though their bladder is empty, some cats strain so hard to
pee that they cause small tears in the bladder wall which produces
hematuria.

but after tests at the vet it's

Sounds like you have a "paint by numbers vet"- that's a vet who goes by the
numbers instead of the cat.

I'd bet dollars to donuts that your cat has feline interstitial cystitis
(FIC) which can't be diagnosed by routine tests. FIC can be caused by a
defect in the glycosaminoglycan (GAG) layer that coats the bladder
epithelium and permits urine to penetrate the urothelium and induce
inflammation. Crystals can also induce inflammation by irritating and
cutting the urothelium.

The best way to treat FIC is feeding your cat canned food-even with a little
added water.  Feeding canned food results in a higher water intake, higher
urine volume, higher water turnover and more frequent urination.  The higher
urine volume dilutes any noxious substances in the urine; the higher water
turnover removes small crystals and calculi before they can irritate the
bladder; and more frequent urination decreases bladder contact time with
urine.  You might also want to try a GAG supplement- such as Cosiquin- to
help repair and maintain the bladder wall.

Good luck,

Phil