The urinalysis - Part II
The urinalysis is an invaluable diagnostic test. In the first part of this two part series (Veterinary Voice April 2009), urine collection technique and interpretation of urine SG were discussed. In the second part of this two part series, urine dipstick and sediment evaluation is examined.
Proper sample handling is important to get the most out of your urinalysis. A minimum of 6 mls of urine should be collected (ideally by cystocentesis). Urine dipstick and sediment exam (with urine at room temperature) should be performed within 20 to 30 minutes of collection. Samples intended for culture should be placed in the refrigerator immediately and submitted for culture within 6 hours.
Urine Dipstick evaluation: a drop of urine should be placed on each test pad with care taken to avoid letting urine run from one test pad to another. Urine pH: results can be variable and dependent on the animal's diet, time of collection, and other factors. Normal pH range is 5.5 to 7.5. For truly accurate results, a pH test meter needs to be used. Ketonuria: may be noted with starvation, diabetes mellitus, or catabolic states. False negative results may occur due to lack of detection of β-hydroxybutyrate (a common ketone body). Bilirubinuria: normal dogs can have small amounts of bilirubin in a concentrated urine sample. The canine kidney can conjugate bilirubin, and also has a low renal threshold for bilirubin. The presence of bilirubinuria in the cat is pathologic. Urobilinogen: levels may be increased with hepatic disease, and absent in cases of biliary duct obstruction. Reagent strips are not always accurate in measuring urobilinogen.. Blood: a positive dipstick result is consistent with hematuria, hemoglobinuria, or myoglobinuria. To determine if hematuria is evident, observe the sediment for intact RBC's. To differentiate myoglobinuria (uncommon) from hemoglobinuria, the ammonium sulfate precipitation test can be performed. Leukocyte esterase reaction: this test was developed for human PMN’s and is not very sensitive in dogs (reported sensitivity of 46%, specificity of 93%). In cats, there is a very high incidence of false positive results. Evaluation of the urine sediment is a more accurate means to detect the presence of WBC’s. Glucosuria: in conjunction with hyperglycemia usually indicates diabetes mellitus. In the absence of hyperglycemia, glucosuria usually indicates familial renal disease or renal tubular injury. Protein: the dipstick predominantly detects albumin. Small amounts of protein (up to 1+ with urine SG > 1.035) are normal. Urine SG should be considered when assessing significance of proteinuria (eg, dilute urine must contain larger amounts of protein before it will be detectable). A 1+ proteinuria in urine SG of 1.006 suggests greater protein loss than a 1+ proteinuria in urine SG of 1.040. False positive urine protein dipstick results may be noted with alkaline urine (pH > 8) and significant hematuria (frank hematuria noted and > 3+ on blood test pad). The sulfosalicylic (SSA) test (routinely run at commercial labs) is another screening test for proteinuria. The SSA test detects albumin, globulin and Bence Jones proteins.
Urine Sediment exam: Preparation: centrifuge urine sample (ideally minimum of 4 mls) at 1000 to 1500 rpm for 5 minutes. The supernatant is pipetted off and a drop placed on refractometer to determine urine SG. A small amount of urine is left remaining (one-half ml) to resuspend the pellet.A drop of the re-suspended pellet is placed on a slide. If desired, a drop of Sedi-stain can be added to an additional separate drop of urine (casts and crystals are best visualized on the unstained preparation; cells, organisms and sperm are best seen on the stained preparation). Turning down the microscope condenser or light source allows for the best viewing of urine sediments. The number of casts is recorded per low power field (lpf), and the numbers of red blood cells, white blood cells, and epithelial cells are recorded per high power field (hpf). The following parameters are evaluated: Mucus: small amounts of mucus can normally be found in the urine due to genital secretions. Cells: WBC's: normal: 0 – 3/hpf via cystocentesis. Increased numbers indicate infectious or inflammatory process. RBC's: normal: < 0 – 3/hpf via cystocentesis. Large numbers of RBC’s indicate hemorrhage (trauma, uroliths, neoplasia, coagulopathies, etc.). Epithelial cells: small numbers are normal. Squamous cells come from vagina or urethra, transitional cells from the kidney to bladder. Increased numbers may be noted with infection, inflammation, or neoplasia, Casts: are cylindrical molds of proteins and cells that form in the renal tubular lumen. Small numbers of hyaline and granular casts are normal. Hyaline casts are composed primarily of mucoprotein (Tamm-Horsfall mucoprotein) secreted by the renal tubules. Large numbers may occur in association with proteinuria. Cellular casts may be composed of WBC’s (associated with infection or inflammation), RBC’s (hemorrhage), or epithelial cells (from renal toxic or ischemic insults). Granular casts form as cellular casts undergo degeneration, or from precipitation of filtered plasma proteins. Large numbers indicate tubular damage or proteinuria. Waxy casts: form from long-standing granular casts and suggest intrarenal stasis. Fatty casts are granular casts that contains lipid granules. These may be noted with the nephrotic syndrome or diabetes mellitus. Crystalluria: struvite, amorphous phosphate, and calcium oxalate crystals may normally be found in the urine in small amounts. Delay in evaluation of urine sediment (> 20 minutes) or storage in the refrigerator can artifactually result in crystal formation. Bacteria: the presence of bacteria in urine may be normal or abnormal depending on how the urine sample is obtained (small bacterial numbers may be noted in free catch samples, no bacteria should be seen in cystocentesis samples). The absence of bacteria on urine sediment exam does not rule out infection (as small numbers of bacteria may not be easily visualized under the microscope, particularly if the urine SG is dilute). Submission of urine samples for culture is important in assessing for possible UTI’s for this reason. Urine for culture should be placed in the fridge and cultured within 6 hours for accurate results.
The urinalysis is an invaluable, readily obtainable diagnostic specimen. Evaluation of the urine SG, dipstick evaluation and sediment exam can aid in assessment of hydration status, allow for the early detection of renal insufficiency, help determine if inflammation or infection is present in the urinary tract, and yield important clues in the evaluation of other organ systems.
Bullet Points
Urinalysis dipstick and sediment exam should be performed within 20 – 30 minutes of collection Urine dipstick evaluation gives valuable information about several organ systems
Not all parameters on the urine dipstick are equally reliable in regards to resultsUrine intended for culture should be placed in the refrigerator and submitted for culture within 6 hrs
Urine sediment exam allows for the detection of renal tubular damage as well as evidence of infection and inflammation of the urinary tract
1
Wendy Yaphe DVM, Dipl ACVIM Aspen Meadow Veterinary Specialists