Hi, about a year and a half ago, my kitty had the same symptoms. A trip to the Vet with a senior bloodpanel and urinalysis confirmed a diagnosis of diabetes melllitus. She is on Lantus insulin and doing very well. I check her blood glucose daily. With the difficulty getting a urine sample, what about having the Vet do a cystocentesis, a procedure in which an uncontaminated urine specimen is removed from the urinary bladder using a needle.
Good luck today at the Vet.