Quantitative bacterial aerobic urine culture with antimicrobial susceptibility testing is the gold standard for diagnosing urinary tract infections. However, it requires several days of incubation, which can result in increased morbidity and mortality in critically ill populations with urinary tract infections (UTIs). This study evaluated Gram stain as a way to identify bacteriuria in dogs.

Gram stain demonstrated increased sensitivity (96% vs 76%), specificity (100% vs 77%), positive predictive value (PPV; 100% vs 83%), and negative predictive value (NPV; 93% vs 69%) compared with routine examination of unstained sediment, which is comparable to reports in human medicine. Gram stain is rapid, easy, and inexpensive to perform. Because Gram stain demonstrated high NPV and PPV, it has excellent predictive values over a range of disease prevalence. Although not a replacement for urine culture, Gram stain offers detection of bacteriuria superior to routine sediment and represents an effective point-of-care test allowing for more rapid identification of infection. Gram stain should be considered part of a routine urinalysis in dogs with active urine sediment or clinical suspicion of UTI.

The results of this study are similar to those of a previous study that evaluated a modified Wright stain of air-dried urine sediment for detection of bacteriuria in dogs. In both cases, staining the air-dried urine sediment improved the detection of bacteriuria compared with routine unstained, wet-mounted, urine sediment preparations. Although the cost to perform is minimal, the extra time and labor required may limit use. Compared with the Wright stain, the Gram stain has the ability to further classify bacteria as Gram positive or Gram negative, which may allow for improved empirical antibiotic treatment while waiting for the bacterial culture and susceptibility results. This hypothesis, however, was not tested in the current study.—Gregory F. Grauer, DVM, MS, DACVIM

Related Article: Recurrent Urinary Tract Infection

Comparison of routine urinalysis and urine Gram stain for detection of bacteriuria in dogs. Way LI, Sullivan LA, Johnson V, Morley PS. JVECC 23:23-28, 2013.