Urine strips: Maximizing the diagnostic value

Apr 01, 2009

Most diagnostic reagent strips used to perform routine urinalysis in veterinary laboratories were designed for human use. Although they do provide useful information to evaluate urine samples from animals, the results obtained with several diagnostic urine strips are unreliable.

Are you familiar with the limitations of the specific brand of diagnostic reagent strip used in your hospital?

The following list summarizes the limitations of most reagent strips and offers suggestions to maximize their diagnostic value:

1. URINE SPECIFIC-GRAVITY values of dogs and cats obtained with reagent strips are unreliable.

SUGGESTION: Use a properly calibrated refractometer to determine the values.

2. URINE PH test pads are designed to measure pH values to within 0.5 units. Because pH values usually are measured on a logarithmic scale, these strips often are too insensitive for reliable evaluation. Also, to prevent a false reduction in pH values when using multiple test reagent strips, prevent runover from the highly acidic protein test pad to the urine pH test pad by holding the strip horizontally.

SUGGESTION: Use a pH meter to determine pH results, especially when trying to measure relatively small changes in urine pH.

3. GLUCOSE TEST PADS contain labile enzymes (glucose oxidase and peroxidase). If the enzymatic action of these proteinaceous enzymes is impaired, test results will be unreliable.

SUGGESTION: Become familiar with the expiration date on the container. The life of enzymes in an unopened container of urine reagent strips may be prolonged by freezing the package. Check with the manufacturer for details.

4. BILIRUBIN REAGENT PADS are unreliable as screening tests in dogs because of a high percentage of false positive and false negative results. In contrast, positive bilirubin test results in cats usually are indicative of some underlying disease.

SUGGESTION: Consider positive or negative urine bilirubin test results in conjunction with other clinical findings.

5. THE OCCULT BLOOD TEST PAD may detect red blood cells, free hemoglobin or myoglobin.

SUGGESTION: Evaluate positive test results in conjunction with evaluation of urine sediment and other clinical findings.

6. PROTEIN TEST PADS routinely incorporated into multiple test reagent strips are based on a phenomenon called the "protein error of indicator dyes." These tests are approximately twice as sensitive to albumin compared to globulin, and three times more sensitive to albumin compared to muco-proteins. In addition, false positive results may occur if the urine pH is very alkaline.

SUGGESTION: Compare questionable results to another test method, such as the turbidometric sulfosalicylic acid test. Evaluation of urine protein-creatinine ratios may be helpful because they are a more reliable method to determine the magnitude of pathologic proteinuria.

7. UROBILINOGEN TEST PADS have not been useful in the routine evaluation of canine and feline urine.

SUGGESTION: Do not rely on uro-bilinogen test pads to screen patients for hemolytic disorders, hepatic disorders or patency of bile ducts.

8. NITRITE TEST PADS are used as an indirect indication of bacteriuria in humans. However, they provide uniformly false negative results in dogs and cats. False negative results associated with nitrite tests are associated with interference caused by ascorbic acid normally present in canine and feline urine.

Table 1: Correlation of some common urinary-tract diseases with expected proteinuria and urine-sediment findings
SUGGESTION: Evaluate urine sediment and bacterial cultures to rule in or rule out bacterial urinary-tract infections.