Today's Veterinary Practice

MAY-JUN 2014

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May/June 2014 Today's Veterinary Practice 89 URinaLysis in coMPanion aniMaLs PaRT 2: eVaLUaTion of URine cheMisTRy & sediMenT Today's Technician | tvpjournal.com samples should not be directly exposed to light. D i s c o l o r a t i o n of urine (due to hemoglobinuria and myoglobin- uria) causes non- speci f ic color change in the bilirubin reagent pad, which inter- feres with read- ing the test strip. MICROSCOPIC EXAMINATION OF URINE SEDIMENT After preparation of the urine sediment slide (see How to Prepare Urine for Microscopic Examination), micro- scopic examination of the sediment is performed with the sub-stage condenser of the microscope lowered. The initial scanning of the slide is performed on low power (10×), which enables the examiner to evaluate the quantity of material present and quality of the sample prep- aration. Using the fine focus while scanning, the examiner can assess particles suspended in different planes of the fluid. Examination at high power (40×) enables the examiner to evaluate cell number and morphology, and identify casts and crystals. Each of these elements may be counted by aver- aging the number of elements in 10 fields. The cells, casts, and crystals are reported as the average number per high- power field (HPF) or low-power field (LPF). Cells Red Blood Cells. Up to 5 red blood cells (RBCs) per HPF may be present in healthy animals. RBCs are small, bicon- cave, and without internal structure (Figure 6). Due to their biconcave structure, RBCs look like a "donuts" when manipulating the fine focus of the microscope. RBCs exposed to urine for prolonged periods become acid, less sensitive to acetone, and does not detect beta-hydroxybutyric acid; therefore, it often under- estimates the amount of ketones present. • Acetoacetic acid decomposes to acetone and, because acetone is volatile, it diffuses into the atmosphere. False negatives or false low ketone concentration may result if urine samples are not quickly analyzed. Ketonuria indicates a shift from carbohydrate metab- olism to fat metabolism. In small animals, this shift is most commonly associated with ketosis second- ary to diabetes mellitus, but starvation also results in increased ketones. These conditions are characterized by metabolic demands that exceed the level that can be provided by carbohydrate metabolism. Occult Blood The test strip pad for blood in urine detects heme- containing substances through an enzymatic chemi- cal reaction that results in a color change proportion- al to the amount of substance present. The heme may be from hemoglobin or myoglobin. • Free hemoglobin is from lysed erythrocytes or intact erythrocytes. • Myoglobin, a protein present in muscle cells, can be detected when extensive muscle damage or necrosis has occurred. • Positive occult blood results are most commonly associated with hematuria (Table 2) rather than hemoglobinuria. Occult blood reactions should be interpreted along with urine sediment findings. Presence of red cells in the sediment indicates that hematuria is causing the occult blood. However, if urine is dilute or alkaline, intact red cells may not be detected because they may have lysed. Bilirubin Bilirubin is not present in the urine of most domestic animals, except the dog. Small amounts of bilirubin may be detected in healthy dogs, particularly in con- centrated urine. 1,3 Bilirubin is a breakdown product of hemoglobin produced by senescent red cells being removed from gen- eral circulation. Conjugated bilirubin can pass through the glomerulus and be excreted in the urine. The renal threshold for bilirubin is low in most ani- mals, particularly dogs. The test strip pad for biliru- bin detects it by chemical reac- tion, producing a color change that indicates the amount of bilirubin present. Because bili- rubin is degraded by ultravio- let light, prior to analysis urine TabLe 2. causes of hematuria • coagulopathy • collection method (catheterization or cystocentesis) • drugs (ie, cyclophosphamide) • estrus • Genital tract hemorrhage • inflammatory renal disease • Kidney neoplasia • Kidney or ureter calculi • Polycystic kidneys • Prostatic disease • sterile inflammation • Trauma • Urinary bladder tumor • Urinary tract infection • Urolithiasis Figure 6. Hematuria: Red blood cells (thick arrow) and white blood cells (thin arrow) Figure 7. Transitional cells Figure 8. Squamous cells TVP_2014-0506_TT_Urinalysis-Part2.indd 89 5/25/2014 7:28:48 PM

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