Urinalysis: What is your interpretation?

Urinalysis: What is your interpretation?

Sep 01, 2007

Urinalysis is one of our most important clinical diagnostic tools. Unfortunately, most diagnostic reagent strips used to perform routine urinalyses in veterinary laboratories have been designed for human use.

Although these reagent strips provide useful information when used to evaluate urine samples collected from animals, test results obtained with portions of several urine diagnostic reagent strips are unreliable.

Are you familiar with the limitations of the specific brand(s) of diagnostic reagent strip used in your hospital? Here are the limitations of most reagent strips, along with suggestions to minimize errors associated with them:

Osmolality and specific-gravity values for adult dog, cat and human urine
1. URINE SPECIFIC GRAVITY: values of dogs and cats obtained with reagent strips are usually unreliable. Because the highest value that these reagent pads can detect is 1.025 to 1.030 (Table 1), they are unsatisfactory to detect adequate renal concentrating capacity in dogs or cats.

TIP: Use a refractometer to determine urine specific-gravity values.

Factors influencing numbers of casts, white cells, red blood cells or crystals in urine sediment of dogs and cats
2. URINE PH test pads included with urinalysis reagent strips have been designed to allow measurement of pH values to within +/- 0.5 units. This is satisfactory for routine evaluation of most patients. However, these test pads will not permit precise determination of urine pH values. Also, care should be used to prevent runover from highly acidic protein test pads to adjacent urine pH test pads to prevent a spurious reduction in pH.

TIP: Use a (portable) pH meter to confirm results, especially when trying to measure relatively small changes in pH units.

3. GLUCOSE TEST PADS typically contain labile enzymes (glucose oxidase and peroxidase). If these proteinaceous enzymes become inactive, false-negative results will result.

TIP: Become familiar with the expiration date listed on the container by the manufacturer. The viability of enzymes in an unopened container may be prolonged by freezing the package. (Check with the manufacturer for details.) Evaluate test results in conjunction with evaluation of tests for glucose by a different method (such as copper reduction method).

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

TIP: 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.

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

6. PROTEIN TEST PADS are approximately twice as sensitive to albumin compared to globulin, and three times more sensitive to albumin compared to mucoproteins. In addition, false-positive results may be obtained if the urine pH is very alkaline.

TIP: Compare questionable test results to results obtained by another test method, such as the sulfosalicylic acid (turbidometric test). Determination of urine protein-creatinine ratios are a more reliable method to determine the magnitude of pathologic proteinuria.