2.1.1. Diagnosis of Uncomplicated UTIs
Proper and timely diagnosis is critical for management of UTIs. Proper diagnosis allows for determination of both the need for antimicrobials and optimal drugs.
As discussed above, clinical signs are nonspecific and should not be used alone for diagnosis of UTI. Rather, the presence of clinical abnormalities should indicate the need for further testing.
Sediment analysis alone is inadequate for diagnosis of UTIs because of problems regarding the variable quality of interpretation, stain contamination, and false positive results from bacteriuria in the absence of clinical infection. Hematuria and proteinuria are often present with a UTI, but they are nonspecific and may be caused by noninfectious conditions. The presence of pyuria and bacteriuria does, however, provide supporting evidence of a UTI. Sediment analysis is a useful adjunctive measure to consider in conjunction with clinical signs and culture results. All individuals performing analysis should be proficient at the technique, ideally through formal training and ongoing education and quality control.
Complete urinalysis, including urine-specific gravity, urine glucose level determination, and examination of the sediment for crystalluria is considered a minimum database for evaluation of suspected UTI and may be helpful to investigate underlying causes of infection, if present.
Aerobic bacterial culture and susceptibility testing should be performed in all cases, to confirm the presence of infection, identify the presence of resistant bacteria that may not respond to initial therapy, to help differentiate reinfection from relapse should a UTI return, and to provide the clinician with guidance regarding the most common bacteria causing UTI in their practice and local susceptibility patterns.
Cystocentesis should be used for sample collection. Catheterized samples can be evaluated for culture but cystocentesis samples are preferred. Free-catch (midstream voiding or manual expression) samples should not be used. It is imperative that a quantitative culture be performed.
Urine samples for culture and susceptibility testing should be refrigerated immediately after collection and submitted to the laboratory as quickly as possible. Results of samples that take 24 hours or more to reach the laboratory should be interpreted with caution because of the potential for both false positive and false negative results, particularly if a urine preservative was not used. Testing of refrigerated samples greater than 24 hours old is acceptable if samples contain a urine preservative; otherwise, retesting is recommended.
The use of specific urine transportation tubes is recommended, provided they are appropriate for the volume of urine that is collected. The use of new or alternative techniques or materials intended to facilitate successful culture such as inoculation of “urine paddles” in clinics is a reasonable alternative to traditional sample collection approach to try to optimize bacterial recovery. The preferred approach is for paddles to be inoculated in clinics and promptly submitted to a diagnostic laboratory for incubation and subsequent testing.
The use of traditional culture methods in clinics may be a reasonable alternative to submission to outside laboratories. In-clinic testing can minimize the impact of sample deterioration that occurs between sample collection in the clinic and processing at the laboratory and can be cost-effective for screening purposes. However, bacterial isolation should only be attempted in clinics with appropriate laboratory facilities, proper equipment, proper biosafety level 2 (BSL-2) containment and waste management, and adequately trained individuals. Quantitative culture should be performed. Incubation of urine paddles in the clinic can be performed, but this approach has the same biosafety requirements as incubation of culture plates.
Identification and susceptibility testing must only be performed if there is adequate biocontainment and properly trained staff. Only protocols that include reference strains for quality control testing and have been standardized by an appropriate organization (i.e., Clinical and Laboratory Standards Institute (CLSI), European Union Committee on Antimicrobial Susceptibility Testing (EUCAST), or equivalent) should be followed.
If culture is being performed to screen samples and send isolates (on plates, paddles, swabs, or any other approach) to a diagnostic laboratory for subsequent testing, clinicians must contact their laboratory to determine if those items will be accepted. In all cases in which isolates are shipped, regional regulations must be followed regarding shipment of bacteria. If a veterinary clinic is unable to satisfy BSL-2 containment and properly ship isolates, the use of these techniques is discouraged.
The advantage of quantitative culture techniques lies in the availability to determine the level of bacterial growth (colony counts), which can be used in interpreting the relevance of results. For samples collected by cystocentesis, any level of bacterial growth may be significant, although samples from a UTI typically contain ≥103 colony forming units (CFU)/mL [6]. The colony count and the identity of the organism isolated should be considered in all situations. Small numbers of minimally pathogenic skin commensals (i.e., coagulase negative staphylococci) likely represent contamination.
For samples collected via catheter, bacterial counts ≥104 CFU/mL in males and ≥105 in females are typically considered significant. Samples with lower counts should be interpreted with caution and ideally repeated prior to treatment to confirm the same organism can be demonstrated. Samples obtained in male dogs by catheterization are usually adequate so long as proper sterile technique was performed to obtain the sample. Positive cultures obtained from catheterized female dogs should be confirmed with a cystocentesis unless medically contraindicated.
Although it has been suggested that bacterial counts of greater than or equal to 105 CFU/mL in dogs and 104 CFU/mL in cats are significant for free-catch samples [6], the potential for high-level contamination is present and therefore results from free-catch samples are not considered diagnostic. Cystocentesis should be performed to confirm positive culture results from free-catch samples, unless medically contraindicated.
Susceptibility testing should be performed according to accepted standards, such as those published by the CLSI or EUCAST, or another internationally recognized public standard. The interpretive criteria for susceptibility testing and breakpoints for systemic infections also apply to UTIs. Those drugs for which urinary tract-specific breakpoints have been provided are limited to ampicillin or amoxicillin in dogs (≤8 μg/mL), amoxicillin-clavulanate in dogs and cats <8/4 μg/mL), and nitrofurantoin.