Cats don't get the frequent UTIs that dogs do, and more frequently get FLUTD, which is a blockage due to crystals or stones in the urine. Since you don't have a diagnosis I'm a little nervous about you just giving antibiotics and hoping for the best.
The gold standard for diagnosing a urinary tract infection is the same for dogs and cats: Take a sterile urine sample by cystocentesis (a free catch is a poor second choice, as it's usually contaminated by contact with the outer urinary tract and skin and fur), and do a urinalysis and urine culture. If the symptoms are very distressing, the cat can be started on a "good guess" antibiotic before the results of the culture are back (it takes three days), and then you can switch to an indicated antibiotic if the culture and sensitivity shows that bacteria is not sensitive to the one you're using.
If it's the right diagnosis and the right antibiotic, you should see a dramatic improvement in symptoms within 24-48 hours.
Once the cat has started on antibiotics, you can't do a urine culture, because even the wrong antibiotic can prevent the growth of bacteria in culture ... and not be enough to cure the infection. So it's best to always, always, always get that urine sample prior to starting antibiotic therapy.
It's best to do a follow up urine culture after the antibiotic therapy is done to be absolutely sure you got the infection. Most recurrent urinary tract infections are actually just the SAME infection that was never fully eradicated.
And the only way to know if your cat is having repeated urine infections, which can indicate an underlying problem such as thyroid disease, a structural defect, etc., or having a relapse of the same infection that's never really eradicated, is by doing these urine cultures. Without that, you're shooting blind and potentially giving an inappropriate treatment, wasting time and having your kitty suffer needlessly.
If this were my cat, I'd do an xray if you didn't already, and discuss with your vet if there is a way to get a urine culture done even though you have started antibiotics. There are supposedly tubes with little crystals in them that absorb the drug residue in the urine, however, in recent years I haven't had any luck locating them. Your vet might want to phone the lab that does his urine cultures and ask them what they suggest. But I wouldn't just wait, especially if after 48 hours in the antibiotics the cat isn't almost completely normal.
Good luck!
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Christie Keith
Caber Feidh Scottish Deerhounds
Holistic Husbandry since 1986
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