Effect of refrigeration of clinical canine urine samples on quantitative bacterial culture

OBJECTIVE To examine the effect of 24 hours of refrigeration on urine samples collected from dogs with signs of urinary tract infection (UTI). DESIGN Prospective cross-sectional study. ANIMALS 104 dogs with signs consistent with UTI that had a urine sample collected via cystocentesis as part of their diagnostic workup. PROCEDURES A 1-mL aliquot of each urine sample was refrigerated at 5°C for 24 hours in a plain glass tube, then processed for quantitative bacterial culture (QBC). A 0.5-mL aliquot was added to 3 mL of tryptic soy broth (TSB) and refrigerated at 5°C for 24 hours, then processed for QBC. The remaining portion was immediately processed for QBC, with results reported as numbers of bacterial colony-forming units (CFUs). Sensitivity of the QBC for detection of bacteria (and therefore UTI) was determined for sample refrigeration in the 2 conditions, compared with immediate processing (reference standard). RESULTS Bacterial growth was identified in 35.6% (n = 37), 33.7% (35), and 31.7% (33) of the immediately processed, refrigerated, and refrigerated-in-TSB urine samples, respectively. Sample refrigeration without TSB resulted in no significant difference in CFU counts relative to immediate processing; however, the sensitivity of this method was 95% (35/37). Sample refrigeration with TSB resulted in significantly lower CFU counts, and sensitivity was only 89% (33/37). CONCLUSIONS AND CLINICAL RELEVANCE Canine urine samples collected for bacterial culture should be immediately submitted for testing. Although CFU counts for refrigerated and immediately processed samples were statistically similar in this study, sample refrigeration in enrichment broth resulted in imperfect sensitivity for UTI detection and is not recommended.