Hi Grace
It must be frustrating! You are wise to maintain your kidney health.
I am assuming you have an incontinent urostomy (meaning no new bladder was created and you wear a pouch). With this type of urinary diversion, infection is one of the frequent complications. Possible causes could include urine refluxing (going backwards and up) to the kidneys, and kidney stones. There may be other reasons, including pouch hygiene.
Have you seen your urologist rather than your general practitioner for follow-up? Imaging the urinary tract system could help diagnose or rule out issues.
A couple of general pointers helpful in preventing infection:
Be sure to wash hands thoroughly before changing your pouch. You may also want to change your pouch system more frequently (every 3-4 days) as bacteria can grow in the pouch. I had one physician who advocated putting a small amount of Betadine in the pouch to kill bacteria, I personally haven't read studies to prove this helped though the literature may be out there.
Staying hydrated is important, though one study suggests forcing too much fluid may nullify the good effects of antibiotics. Adding 1-2 glasses of cranberry juice or taking cranberry pills may help to acidify your urine; this has proven beneficial to preventing certain bacteria from clinging to the urinary organs. Vitamin C in large doses may also be of benefit, but would check with a urologist first as it can also cause issues with stone formation.
When giving a urine specimen an ostomy nurse or urologist should catheterize your stoma in a sterile manner to get an accurate specimen. It should not be taken from the pouch even if it is clean since it is not sterile (though I see this manner of collection a lot!) If catheterization is not possible, I'd suggest removing the entire appliance, and having the nurse or physician clean the stoma with sterile technique, and place a sterile cup under the stoma to collect the urine. (Yes, it takes a while!)
I recall a couple of patients years ago that were on long-term antibiotics as a preventive measure due to frequent infections; it was years ago and not certain why or if this is a frequent practice.
Have you seen the urologist who did your original surgery? Though any urologist could help, your surgeon may have more specific knowledge on how your urinary diversion was constructed...may be beneficial.
Hope some of the ideas will help.