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Urine strips: Maximizing the diagnostic value


DVM360 MAGAZINE


9. LEUKOCYTE TEST PADS give false positive results in most cats in absence of pyuria, and therefore are of no value in this species. Leukocyte test pads frequently give false negative results in dogs, even when pyuria is present. Although the test is specific for WBC in dogs, it is very insensitive.

SUGGESTION: Using standardized technique, urine sediment should be evaluated to detect, characterize and quantify white cells, (along with epithelial cells, renal casts, bacteria, yeast cells or hyphae, parasite ova and crystals). These cannot be detected reliably by macroscopic evaluation of urine with reagent strips.


Table 2: Factors affecting numbers of structures in sediment
Although the normal range of white cells (neutrophils) in urine sediment prepared from a 5-ml. aliquot of urine centrifuged at 2,000 to 3,000 rpm for five minutes has been reported to be zero to three white cells per high-power field (450X) in samples collected by cystocentesis, and zero to eight white cells per high-power field in catheterized or midstream voided samples, several variables that influence white-cell numbers should be considered (Table 2).

Tthere is no absolute cut-off point between upper numbers of "normal" white-cell numbers and lower limits of "abnormal" white-cell numbers.

Variation in results may be minimized by standardizing the procedure of preparing and analyzing urine sediment.

Because knowledge of urine specific gravity provides useful information on the relative concentrations of water and elements in urine sediment, urine sediment results should be interpreted in conjunction with urine specific gravity.

Although various types of diseases (inflammatory, infectious, neoplastic, etc.) may be recognized on the basis of positive findings, they cannot always be eliminated on the basis of negative findings.

For example, 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.


Table 3: Normal vs. abnormal findings in urine sediment
However, since bacteria are more difficult to detect than white cells, pyuria may appear to be unassociated with low numbers of bacteria. Therefore, urine obtained from patients with significant pyuria should be routinely cultured for bacteria.

Another example is interpretation of white-cell casts.

Observation of them indicates renal tubular involvement in the inflammatory process. However, white-cell casts rapidly degenerate to become granular casts.

In our experience, white-cell casts are uncommonly associated with bacterial infections of kidneys.

Therefore, absence of white-cell or granular casts does not exclude renal involvement in the inflammatory process.

The point is that the results of examination of urine sediment must be interpreted in combination with other clinical data, including the physical and chemical characteristics of urine (Table 1).

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.


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