Urinary Tract Infection

Question
I have an 8-year old female cat who had/has her first UTI.  A culture was done and did not show signs of anything. She was put on antibiotic and will take her last dose of Zeniquin 25 mgs tomorrow (one-week total time on antibiotic).

Today I saw her squatting in the corner again, and I saw blood in her urine.  I'm keeping an eye on her by keeping her in a single room (as opposed to running around the house as she usually does) to monitor her passing water.  Question(s):  does this mean the UTI needs another kind of antibiotic? what else could this mean?  This is only the second cat I've owned, and my very first experience with a UTI.

Answer
It is EXTREMEMY rare for a cat to get a urinary tract infection.  If the culture was negative, then your vet should know that there is no bacterial infection.  Most cats have a problem with crystal formation in their urine from too much magnesium in the diet creating struvite crystals.  It is the crystals that cut the lining of the urethra causing pain and blood.  Your vet has special foods to combat this problem.