| Microscopic examination of urine sediment should be part of a routine urinalysis. For centrifugation, 3-5 mL of urine is transferred to a conical centrifuge tube. Urine is centrifuged at 1,500-2,000 rpm for ~5 min. The supernatant is decanted, leaving ~0.5 mL of urine and sediment in the tip of the conical tube. The sediment is resuspended by tapping the tip of the conical tube against the table several times. A few drops of the sediment are transferred to a glass slide, and a
cover slip is applied. Examination of unstained urine is recommended for routine samples. Microscopic examination is performed at 100¥ (for crystals, casts, and cells) and 400× (for cells and bacteria) magnifications. Contrast of the sample is enhanced by closing the iris diaphragm and lowering the condenser of the microscope. Stains such as Sedistain® and new methylene blue can be used to aid in cell identification but tend to dilute the specimen and
introduce artifacts such as stain precipitate and crystals. |
| In an unstained preparation, RBC are small and round and have a slight orange tint and a smooth appearance. Normal urine should contain <5 RBC/field at 400× magnification. Increased RBC in urine (hematuria) indicates hemorrhage somewhere in the urogenital system; however, sample collection by cystocentesis or catheterization may induce hemorrhage. |
| WBC are slightly larger than RBC and have grainy cytoplasm. Normal urine should contain <5 WBC/field at 400¥ magnification. Increased WBC (pyuria) can occur due to inflammation, infection, trauma, or neoplasia. Catheterization or collection of voided urine may introduce a few WBC from the urogenital tract. |
| Transitional epithelial cells, a common urine contaminant derived from the bladder and proximal urethra, resemble WBC but are larger. They have a greater amount of grainy cytoplasm and a round, centrally located nucleus. In a voided urine sample, squamous epithelial cells may be observed. They are large, oval to cuboidal in shape, and may or may not contain a nucleus. Occasionally, neoplastic transitional cells may be observed in an animal with a transitional cell carcinoma.
Neoplastic squamous cells may be observed in an animal with a squamous cell carcinoma. |
|
Casts are elongated, cylindrical structures formed by mucoprotein congealing within renal tubules and may contain cells. Hyaline casts have parallel sides and rounded ends and are composed of mucoprotein. They may occur with fever, exercise, and renal disease. Epithelial cellular casts form from entrapment of sloughed tubular epithelial cells in the mucoprotein; they may be observed with renal disease. Granular casts are thought to
represent degenerated epithelial cellular casts. Waxy casts have a granular appearance, and are thought to arise from longstanding granular casts. They typically have sharp borders with broken ends. Other cellular casts include erythrocyte casts and WBC casts. Erythrocyte casts form because of renal hemorrhage. WBC casts occur because of renal inflammation, as with pyelonephritis. Fatty casts are not common, but can be observed with disorders of lipid metabolism, such as diabetes
mellitus. A few hyaline or granular casts are considered normal. However, presence of cellular casts or other casts in high numbers indicates renal damage, and may be one of the earliest laboratory abnormalities noted with toxic damage to renal epithelial cells (eg, gentamicin, amphotericin B). |
| The presence of bacteria in urine collected by cystocentesis indicates infection. Small numbers of bacteria from the lower urogenital tract may contaminate voided samples or samples collected by catheterization and do not indicate infection. Bacterial rods are most easily identified in urine sediment. Particles of debris may be mistaken for bacteria. Suspected bacteria can be confirmed by staining urine sediment with Gram’s stain; however, aerobic culture is best to confirm a
bacterial urinary tract infection. Rarely,
yeast and fungal hyphae and parasitic ova may be observed in urine sediment. Their presence is not always associated with clinical disease. Parasitic ova observed include
Stephanus
dentatus
,
Capillaria
plica
,
C
felis
, and
Dioctophyma
renale
. Additionally, microfilariae of
Dirofilaria
immitis
may be observed in urine sediment. |
| Many urine sediments contain crystals. The type of crystal present depends on urine pH, concentration of crystallogenic materials, urine temperature, and length of time between urine collection and examination. Crystalluria is not synonymous with urolithiasis and is not necessarily pathologic. Furthermore, uroliths may form without observed crystalluria. Struvite crystals are commonly observed in canine and feline urine. Struvite
crystalluria in dogs is not a problem unless there is a concurrent bacterial urinary tract infection with a urease-producing microbe. Without an infection, struvite crystals in dogs will not be associated with struvite urolith formation. However, some animals (eg, cats) do form struvite uroliths without a bacterial urinary tract infection. In these animals, struvite crystalluria may be pathologic. Struvite crystals appear typically as “coffin-lids” or “prisms”; however, they may
be amorphous. Calcium oxalate crystalluria occurs less commonly in dogs and cats; if persistent, it may indicate an increased risk for calcium oxalate urolith formation. (See also
urolithiasis,
Urolithiasis: Overview and
Urolithiasis: Overview .) However,
calcium oxalate and calcium carbonate crystalluria is common in healthy horses and cattle. Calcium oxalate dihydrate crystals appear as squares with an “X” in the middle or “envelope-shaped.” Calcium oxalate monohydrate crystals are “dumb-bell” shaped. An unusual form of calcium oxalate crystals is typically seen in association with ethylene glycol toxicity (
Ethylene Glycol Toxicity: Introduction). These crystals occur in
neutral to acidic urine. They are small, flat, and colorless, and are shaped like “picket fence posts.” Ammonium acid urate crystals suggest liver disease (eg, portosystemic shunt). These crystals occur in acidic urine and are yellow-brown spheres with irregular, spiny projections; however, they may also be amorphous. Certain species, such as birds and reptiles, and certain breeds of dogs, specifically Dalmatians, can normally have ammonium acid urate crystalluria. Cystine
crystals are 6-sided and of variable size. They occur in acidic urine. Presence of cystine crystals represents a proximal tubular defect in amino acid reabsorption. Cystinuria has been reported to occur in many breeds of dogs and rarely in cats. Dachshunds, Newfoundlands, English Bulldogs, and Scottish Terriers have a high incidence of cystine urolithiasis. Bilirubin crystals occur with bilirubinuria; however, they may be normal in small numbers in dogs. |
| Fat droplets are commonly present in urine from dogs and cats and may be mistaken for RBC. However, they often vary in size and tend to float on a different plane of focus than the remainder of the sediment. They are not considered to be pathologic. |
| Spermatozoa may be observed normally in urine collected from male dogs. |
| Occasionally, plant material may be observed in urine samples collected by voiding. When present, they indicate contamination of the urine sample and are not pathologic. |
