In cats, urinary sediment examinations and urine cultures often yield discordant results. In this study, investigators compared wet-mount and air-dried Wright-stained urinary sediments with quantitative aerobic bacterial culture of urine. Urine specimens (n = 472) from 410 cats were collected by cystocentesis. A complete urinalysis, urine culture, wet-mount unstained specimen, and air-dried modified-Wright (dry-stained) specimen were compared.

Of the 472 samples, 29 (6%) had a positive culture for bacteriuria. Of the 33 bacterial isolates, the 3 most common were Escherichia coli (45.5%), Staphylococcus intermedius (24.3%), and Enterococcus spp (21.2%). Bacteria (≥1/HPF) were observed in 214 of 472 (45.3%) wet-mount unstained samples. Bacteria (≥10/20 OIF) were seen in 30 of 472 (6.4%) dry-stained sediments. Pyuria was reported in 36 of 472 (7.6%) wet mounts. Of these, 10 had cultured positive for bacteriuria and 26 had not. When cytology results for wet mounts were compared with culture results, urine sediment yielded 4.6% true-positive results, 1.5% false-negative, 53.2% true-negative, and 40.7% false-positive. Sensitivity and specificity were 75.9% and 56.7%, respectively. When cytology results for dry-stained specimens were compared with culture results, there were 5.1% true-positive results, 1% false-negative, 92.6% true-negative, and 1.3% false-positive. Sensitivity and specificity were 82.8% and 98.6%, respectively.

In the 29 positive culture samples, the dry-stained method had greater morphologic concordance and lower misclassification rates than the wet-unstained method. The dry-stained method using a modified-Wright stain had improved sensitivity, specificity, and overall test efficacy versus the wet-mount unstained method.

Commentary: The results of urine sediment examination are often used to tentatively diagnose a urinary tract infection, assess the need for urine culture, and/or initiate empiric antimicrobial treatment. Unfortunately, there are often discordant results between the urine sediment examination and actual urine culture results. In most cases, the discordant results are due to small particles (pseudobacteria) that resemble bacteria in size, shape, and movement, which can be misinterpreted as bacteria during examination.

Similar to their previous findings in dogs, the authors showed that an air-dried, modified-Wright (Diff-Quik) stain was more accurate for detection of bacteriuria compared with the standard unstained wet preparation for urine sediment examination. Also similar to previous studies, true bacteriuria in feline urine samples was relatively rare (29/472, or 6.1%) with E coli being the most common isolate, and bacteriuria was most commonly observed in senior (>10 years of age) and female cats. Only one-third of the culture-positive urine samples had pyuria (>5 WBC/HPF); therefore, pyuria should not be a criterion for determining the presence or absence of bacteriuria in feline urine samples.—Gregory F. Grauer, DVM, MS, Diplomate ACVIM

Evaluation of modified Wright-staining of dried urinary sediment as a method for accurate detection of bacteriuria in cats. Swenson CL, Boisvert AM, Gibbons-Burgener SN, Kruger JM. VET CLIN PATHOL 40:256-264, 2011.