Because of the relative insensitivity of routine urinalysis for detecting bacteriuria, occult cases of urinary tract infections (UTI) can be overlooked if urine is not cultured. Urine culture is the most sensitive and specific test for diagnosing UTI.

Cultures on liquid urine are useful to diagnose UTI, which may be associated with vaginitis, prostatitis, urolithiasis, pyelonephritis, urethritis, ureteritis, vesicoureteral reflux, congenital disorders (e.g. ectopic ureter), trauma, chronic renal failure, endocrine disorders (e.g. Cushing's disease, diabetes mellitus), and neoplasia.

In a published study of 101 dogs with hyperadrenocorticism, diabetes mellitus, or both disorders, 42% had UTI diagnosed by urine culture. UTI was present in 46% of dogs with hyperadrenocorticism, 37% of dogs with diabetes mellitus, and 50% of dogs with both disorders. E. coli was the most common bacterium isolated, present in 69% of dogs. Clinical signs of dysuria or pollakiuria were present in <5% of the dogs with UTI, and 19% of dogs with positive cultures had neither pyuria or bacteriuria on urinalysis. Urine culture should be included routinely as part of the initial evaluation of dogs with these endocrine disorders regardless of clinical signs or urinalysis findings.

Senior pets often have occult UTI. Of 90 clinically normal geriatric dogs studied at VCA West Los Angeles Animal Hospital in 1998 and 1999, 12 dogs had UTI in the absence of any clinical signs of lower urinary tract disease (LUTD).

Although UTI are an uncommon cause of LUTD in cats, urine culture should be performed in cats with multiple episodes, in those that have perineal urethrostomies or have been catheterized, and those with urine specific gravity <1.035.

Chronic renal failure is a risk factor for UTI in cats, as up to 30% of cats with this disease have UTI. Similarly, according to Dr. Dennis Chew at Ohio State University, cats with perineal urethrostomies have a >50% incidence of UTI. Hence, cats with chronic renal failure or perineal urethrostomies should have urine cultures checked periodically to permit early detection of UTI.

Dogs—Recent evaluation of dog urine cultures indicates that positive cultures become positive by 48 hrs. Negative cultures at 48 hrs remained negative at 72 hrs. Thus, all negative canine urine cultures will be reported as final at 48 hrs.

Cats—A few (~1%) of feline urine cultures become positive at 72 hrs after being negative at 48 hrs. Feline urine cultures will therefore continue to be checked for 72 hrs.

In addition to urine culture, urine colony counts or quantification of urine cultures can be performed to determine the number of bacterial colonies per mL of urine. This is helpful in assessing whether the bacteria isolated and identified on culture are possible contaminants, normal flora, or significant pathogens when urine is not collected by cystocentesis.

To correctly interpret the results of urine cultures, certain collection procedures must be followed:

• Best results are obtained by sterile urine collection, preferably by cystocentesis. The site should be cleaned with alcohol or disinfectant to minimize the risk of contamination by cutaneous flora.
• After cystocentesis, the urine should be placed for culture and/or urine colony count into a sterile container. Be sure to label the tube as "Urine for Culture:. Only a small amount of urine (even 0.5 mL) is sufficient.
• If a routine urinalysis is also required, a separate aliquot of urine should be submitted in a plain red top (not serum separator) tube or specimen cup.
• The animal should be off antibiotic therapy for at least 48-72 hours prior to cystocentesis to prevent suppression of bacterial growth.

Results of urine colony counts are reported as the number of colonies (or the range) per mL of urine along with the identification of the organism and antimicrobial sensitivity. For urines collected by mid-stream catch or catheterization, less than 10,000 per mL is insignificant. Also, mixed bacterial isolations (2 or more different bacteria) are indicative of contamination and do not accurately reflect the urinary tract infection, especially when urine is not collected by cystocentesis. The highest reported value for bacterial counts is greater than 100,000/mL.