What is the significance of pyuria?

Case scenario

A voided urine sample was obtained from a 12-year old spayed female Dachshund as part of an annual evaluation of health status. According to the owners, the dog was in good health. The dog had a history of aerobic bacterial lower urinary tract infections on three occasions during the past five years. The three episodes of lower UTI were caused by three different species of bacteria and were eradicated by appropriate antimicrobial therapy. An underlying cause for the recurrent lower UTI could not be found. The most recent episode of UTI occurred one year previously. Urinalysis of a voided sample collected by the owner revealed: specific gravity = 1.025; pH = 7.5; protein = 1+; leukocyte esterase test for white cells = negative; RBC in sediment = 0 to 1 per highpower field; white cells in sediment = 1 to 3 per high-power field. No other abnormalities were detected. Results of a CBC and serum chemistry profile were also normal. Is this magnitude of pyuria of clinical significance in this dog? What is the basis for your answer? Are additional diagnostic tests or treatment warranted?

Points to consider:

Leukocyte test pads: Leukocyte test pads frequently give false-negative test results in dogs, even when pyuria is present. Although the test is specific for white cells in dogs, it is very insensitive. In contrast, leukocyte test pads give false-positive test results in most cats in absence of pyuria, and therefore are of no value in this species. Therefore, it is important to evaluate fresh urine sediment to determine the presence or absence of pyuria in dogs and cats.

Urine sediment: Using standardized technique, urine sediment should be evaluated for white cells. Significant numbers of white cells (which may be associated with RBC and protein) in a properly collected sample suggest an inflammatory lesion of the urinary tract. The inflammatory lesion may or may not be associated with infectious agents. The question related to our Dachshund is, "What number of white cells is significant?"

Significant pyuria: Although the normal range of white cells (neutrophils) in urine sediment prepared from a 5 milliliter aliquot of urine has been reported to be 0 to 3 white cells per high-power field (450X) in samples collected by cystocentesis, and 0 to 8 white cells per high-power field in catheterized or midstream voided samples, several variables that influence white cell numbers should be considered (Table 1). In this context, it becomes readily apparent that there is no absolute cut off point between upper numbers of "normal" white cell numbers and lower limits of "abnormal" white cell numbers.

Table 1

Additional diagnostic points:

1) Detection of a significant number of bacteria in association with pyuria indicates that the inflammatory lesion is active, and has been caused or complicated by bacterial infection. However, since bacteria are more difficult to detect than white cells, pyuria may appear to be unassociated with low numbers of bacteria (Table 2). Therefore, urine obtained from patients with significant pyuria should be cultured routinely for bacteria.

Table 2

2) A positive urine culture in the absence of pyuria should arouse suspicion that the bacteria are contaminants. However, bacterial UTI can occur without detectable concomitant pyuria in patients given glucocorticoids, those with hyperadrenocorticism and those who are immunosuppressed. Therefore, absence of pyuria should not be interpreted as unequivocal proof that bacterial UTI can be excluded.

3) Sometimes urine sediment appears to contain bacteria, yet urine cultures are sterile. There are several possible explanations for this phenomenon (Table 3).

Table 3

4) Observation of white-cell casts indicates renal tubular involvement in the inflammatory process. However, white cells in casts rapidly degenerate to become granular casts. In our experience, white-cell casts are uncommonly observed in patients with bacterial infections of kidneys. Therefore, absence of white-cell or granular casts does not exclude renal involvement in the inflammatory process.

Interpretation of case scenario

A number of factors may have influenced the number of white cells in this Dachshund's urine sediment. Because the sample was collected by the owner and therefore probably not fresh, a variable number of white cells might have disintegrated following collection. Likewise, the alkaline pH of urine would enhance white cell disintegration. Because the sample was collected by the owners during the voiding phase of micturition, some white cells could have originated from the genital tract. With these factors in mind, what would you do next?

We elected to repeat the urinalysis on a sample collected by cystocentesis. The results were: specific gravity = 1.035; pH = 6.5; protein = 1+; leukocyte esterase = negative; RBC in sediment = 2 to 4 per 450X; white cells in sediment = 5 to 10 per 450X. Culture of an aliquot of the cystocentesis sample revealed a significant number of Escherichia coli. These results confirm asymptomatic recurrent bacterial urinary tract infection. The site(s) of this UTI cannot be determined on the basis of available information. The dog was subsequently treated with an antimicrobic based on susceptibility test results. Bacterial culture of a urine sample collected by cystocentesis three days later resulted in no growth, although pyuria was still present. A urine sample collected by cystocentesis two weeks after initiation of therapy was normal.

Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.