Urinary tract infections are often provisionally diagnosed on the basis of clinical signs, physical examination findings, and results of urine sediment evaluation. Definitive diagnosis can only be made with quantitative bacteriologic culture of urine. However, because of expense, time requirements, and lack of laboratories that can provide quantitative cultures, many veterinarians rely on microscopic examination of urine sediment. This study compared the results of quantitative bacteriologic culture of urine with unstained, wet-mounted preparations and modified Wright-stained samples of urine sediment. A total of 459 urine samples were collected by cystocentesis. The wet-mounted slides were prepared by placing 1 drop of sediment on a slide and placing a coverslip on top. Ten fields were examined at 400×, and the number of leukocytes, RBCs, fat droplets, and bacteria per view were recorded. For the stained samples, 1 drop of urine sediment suspension was placed on a clean glass slide, allowed to air dry, and then stained. Slides were examined at 1000×, and the number of bacteria in 20 fields was counted. Counts were classified as none, occasional (1 to 4), few (5 to 9), moderate (10 to 20), and many (> 20).
Results: 74 (16%) of the urine samples grew bacteria on aerobic culture. Bacteria were observed in 152 (33.1%) of the wet-mounted preparations. There was a high rate of false-positive results (19.8% of 459), false-negative results (17.6% of 74), and morphologic misclassification of bacteria by this method. Bacteria were seen in 142 (30.9%) of the modified Wright-stained slides. This method appeared to be an excellent screening test for detection of bacteria and their morphologic classification.
COMMENTARY: This study points out that particulate matter in the urine sediment that exhibits Brownian motion is frequently misinterpreted to be bacteria, most often cocci. This may explain the many instances where "bacteria are observed in the urine sediment but no bacteria grow on culture." The rapid and convenient modified Wright stain (Diff-Quik) can more accurately determine whether bacteria are present in urine sediment.
Evaluation of modified Wright-staining of urine sediment as a method for accurate detection of bacteriuria in dogs. Swenson CL, Boisvert AM, Kruger JM, Gibbons-Burgener SN. JAVMA 224: 1282-1289, 2004.