PERFECTING URINALYSIS TECHNIQUES
Barry Mitzner, D.V.M.
“We have every chance of not finding what we do not look for.”
–Yogi Berra
Introduction
The history behind our modern medicine is filled with references to the importance of urinalysis
as a diagnostic tool. Hippocrates, the ancient Greek physician, was the first practitioner to recognize
the relationship between fluid intake and urine output. So, too, did the ancient Hindus utilize a crude
assay dubbed “the honey urine test” as a means for diagnosing Diabetes Mellitus. Medieval
physicians also got into the act, often employing a complicated urine distillation process in the
diagnosis of everything from broken bones to brain tumors! Despite the fact that our analytical
methods have improved vastly over time, it remains paradoxical that many veterinarians still haven’t
developed an appreciation for the value of this simple, inexpensive, and informative test.
Routine testing of urine specimens; i.e. urinalysis (or urinanalysis, whichever you prefer…)
encompasses four segments: gross observation, specific gravity determination, biochemical analysis,
and microscopic evaluation. Because urinalysis results should always be interpreted as a whole and
each segment bears upon the interpretation of every other, it is imperative that no segment or
segments be omitted.
Specimen Quality
Urine is a highly unstable biological substance. As such, one can expect the best results to be
obtained only from freshly collected specimens. While various urine preservatives and preservative
methods can be used, one should keep in mind that all will have some adverse effect upon some
portion of the final results.
The mechanics of the various specimen collection methods have been discussed in extensive
detail elsewhere. While it’s true that one procedure may in some cases yield a better quality specimen
than another, suffice it to say that the collection method of choice for any particular patient should take
into account the information sought versus the potential for iatrogenic trauma to the patient.
Gross Observation
Specimens should be observed for color, appearance and odor. Red-tinged specimens
usually indicate the presence of blood. Green-tinged specimens may be seen with bacteriuria, purple
specimens with the administration of certain pharmaceuticals, and brown or dark brown specimens
with autoimmune anemia, liver disease, or muscle necrosis. Clear, water-like specimens often
suggest a loss of urine concentrating ability. Bacterial overgrowth can result in specimen turbidity
while foamy specimens are often seen with proteinuria due to excessive renal loss. An acetone-like
smell to the specimen can indicate ketosis while a foul odor may suggest bacterial overgrowth.
Specific Gravity
In the normal patient, urine is generally excreted in concentrated form; i.e. >1.030. This finding
usually indicates to the diagnostician that the kidneys are functioning properly since one of the typical
early signs of renal disease is a loss of concentrating ability and the production of a dilute
(isotheunric) specimen. A very dilute specimen (<1.008 ) can be an indicator of mineralocorticoidrelated disease or other malady or it might just be a routine finding in a normal patient. Clinicians
should keep in mind that the random aberrant specific gravity reading has little to no significance;
especially in an otherwise clinically normal patient. Abnormal specific gravity results which occur
repeatedly, however, are cause for concern and further investigation.
Urine specific gravity is most easily measured with a refractometer, preferably one that’s
temperature compensated. A temperature compensated unit manufactured by Leica Microsystems for
veterinary use is currently the only one available with extended ranges for animals. Urinometers (a
type of hydrometer) may also be used to measure urine specific gravity although precise testing will
require a larger volume of specimen and a keen eye. Some brands of urinalysis strips also include
reagent pads which change color in response to specific gravity; however, in the author’s experience
these values are less reliable than those determined by refractometric methods.
Biochemical Analysis
With regard to the use of urinalysis test strips, some basic rules must be followed in order to
be assured of meaningful results. The test strip package insert should be thoroughly read and
reviewed prior to commencing any patient testing. The insert will not only describe the recommended
testing procedure, but will also clearly document optimum measurement times for each color block,
analytical limitations for each reagent system, interfering factors, storage recommendations and
stability characteristics, and gross indications of product damage. Such vital information can vary
from one manufacturer to another, so don’t assume that all strips are alike!
As mentioned previously, some test strips made for the human market include indicator pads
for specific gravity. These, as well as test pads for leukocytes and nitrite, have demonstrated little
utility when used with animal specimens since both false positive and false negative readings may
occur clinically.
One of the more common biochemical interferences we’ve observed with small animals can
result from the presence of ascorbic acid which is produced endogenously in these species. When
present in high enough concentration, ascorbic acid can interfere with urine glucose readings
(resulting in falsely lowered or negative values) and mask the presence of blood and/or hemoglobin.
While some manufacturers have modified the chemical composition of their reagent systems to
reduce interference by ascorbic acid, it would appear that no manufacturer has yet to completely
eliminate the potential problem.
Urine Microscopy Basics
One of the longstanding problems associated with examination of urine sediment has been the
variability of results which can occur secondarily to variations in test protocol. The method used for
specimen collection, the volume of specimen to be spun down for sediment prep, the amount of time
and the speed at which that specimen is centrifuged, the volume of solute in which the sediment pellet
is resuspended and the thickness of the slide preparation will all affect final results, both qualitatively
and quantitatively. To eliminate or at least reduce the profound effects of these variables, this author
recommends that practitioner laboratories incorporate one of the disposable, standardized urine
sediment evaluation systems (KOVA System; Hycor Biomedical) into the testing protocol. In addition
to improving precision, operators will find that these specialized systems will enhance their recognition
of the various sediment elements. Adding palatability to an otherwise “dirty” procedure is just an
added bonus.
The urinary sediment of animals may contain an array of substances or elements. These
elements may include metabolic products of the kidney (crystals), blood cells or urogenital tract
epithelia, sperm, or proteinaceous substances originating in the kidney (casts). In addition,
exogenous elements such as bacteria, fungi, and parasites may be found. Finally, non-pathogenic
artifacts, therapeutic or incidental in origin, may also be seen, underscoring the importance of practice
in recognizing the various elements for what they are. An easy way to remember these groups of
elements is to think of them as the “4 C’s”:




Cells
Casts
Crystals
“Critters”
Cells
There are 3 main types of epithelial cells found in urine sediments of which all are derived from
the lining of the kidney and lower urinary tract. Squamous epithelial cells originate from the very distal
portions of the tract and may be present in variable numbers depending upon the method of
collection. Squamous cells typically appear as large, flattened, and straight-sided with pyknotic
central nuclei. Their presence, even in large numbers, is rarely significant; however, malignant forms
can occur with squamous cell carcinoma of the urinary tract.
Lining the renal pelvis, ureter, urinary bladder, and proximal urethra are cells referred to as
transitional cells. These cells are distinguished from squamous cells by their polyhedral to spherical
shape, large, centrally-placed spherical nuclei, and balloon-like appearance. While small numbers of
transitional cells may be a normal occurrence, large numbers may suggest inflammation, neoplasia,
or trauma.
Tubular epithelial cells, so named because the are derived from the kidney tubules, are
perhaps the most clinically important of the observed cells in that primary kidney disease may result in
excess numbers of these cells being sloughed into the urinary sediment. Tubular epithelial cells can
be similar in size to transitional cells, sometimes making the two difficult to distinguish
morphologically. Differences between the two include the finding that tubular cells tend to have large,
vesicular, eccentrically-positioned nuclei, polyhedral shape, decreased nuclear to cytoplasmic ratio,
and occasionally villous borders. More importantly, tubular cells do not have the “balloon”
appearance so typical of transitional cells.
Of the peripheral blood cells found in urine, the neutrophil is the most frequently encountered,
easily recognizable by its multi-lobulated nucleus. Neutrophils tend to “ball up” in urine sediments as
contrasted with their flattened appearance in blood smears. While small numbers of WBC’s are
common in normal specimens and especially with voided specimens, large numbers can be
associated with inflammation or infection. Since WBC’s may appear at any inflammatory site
throughout the urinary tract, their presence alone does little to localize the lesion.
RBC’s can also appear in sediments though small numbers (0-3/HPF) can be a normal finding.
Large numbers indicate hemorrhage into the urinary tract, but as with the presence of leukocytes,
cannot be used as a localizing finding unless they happen to be a component of a cast. RBC’s are
most easily recognized by their biconcave spherical shape and pigmentation.
Casts
Casts are proteinaceous cylinders which are formed within the kidney itself and are named
according to their microscopic appearance. When cells are visualized within the protein matrix of the
cast, the cast is named according to the predominant cell type observed; i.e. RBC, WBC, or epithelial
although mixed cell casts can also occur.
Hyaline casts are composed of congealed protein and have a colorless, smooth, cigar-shaped
appearance. Since their size and shape is related to the size of the renal tubular lumen, a widening or
broadening of the cast suggests renal tubular dilatation, often viewed as a sign of long-standing renal
disease.
Like hyaline casts, granular casts may be found in both normal and abnormal patients. They
are similar morphologically to hyaline casts although they tend to be more refractile as a result of their
granularity. Granules found within these casts may be fine or coarse and scattered throughout or
localized to a particular area. Granular casts, like hyaline casts, can also take on a broadened profile
with long-standing renal disease.
Waxy casts are thought to represent a final form of granular casts and are the most refractile
of all casts. They are easily recognized by their many small cracks and broken-off ends. While waxy
casts can be a normal finding in small numbers, large numbers usually denote severe and
progressive renal disease with decreased renal transit time. It’s also been proposed that certain cast
forms might actually represent evolutionary stages of the same cast. For example, granular casts may
originate from cellular casts following cell degradation, while waxy casts may represent the final
breakdown and confluence of the finest granules.
Crystals
Crystals are a common finding in most sediment preparations, their presence (or absence)
being influenced by specimen handling, temperature, specific gravity, and pH. The finding of some
crystalline forms may suggest specific disease conditions such as in the case of oxalate crystals
occurring secondary to ethylene glycol (antifreeze) toxicity, while the presence of other forms may be
no more than incidental. It’s important, nonetheless, for those wishing to become good urine
microscopists to be able to distinguish normal crystals from abnormal forms.
Phosphate crystals are among the most commonly observed. They can represent an
incidental finding or they may be associated with a disease process such as urinary tract infection.
Phosphate crystals can be identified easily by their so described “coffin-lid” appearance although
amorphous forms may also be found.
.
Easily recognized by their characteristic hexagonal shape, Cysteine crystals are a byproduct
of aberrant protein metabolism. They can be a frequent finding in certain breeds, especially
dachshunds.
Ammonium biurate crystals are brownish crystals with a rather distinctive “thorn-apple” shape.
Although they are often seen in patients with portal vein anomalies, their presence in small numbers
may be considered a normal finding.
Bilirubin crystals are most commonly described as “clumps of brown needles” which are joined
at the midpoint. They can be a normal finding in dogs or they may be associated with pathologic
bilirubinuria. When found in the urine of cats, bilirubin crystals are always a significant finding.
Certain drugs such as sulfonamides and iodine-containing compounds (used in radio-contrast
studies) can also result in incidental crystalluria. Their presence is noteworthy in that they have the
potential to be confused with primary pathologic forms.
Critters
This final category includes such extra-corporeal elements as bacteria, fungi, and parasites.
Bacterial forms are typically found in association with urinary tract infection. When bacteria are
observed in the absence of inflammatory cells (WBC’s), however, it’s likely that the specimen became
contaminated later on and/or was allowed to incubate for several hours prior to analysis.
Fungal hyphae are a rare finding in urinary sediments and an occasional fungal form may be
viewed as a contaminant. Findings of larger numbers of organisms, especially in conjunction with
inflammatory cells, can be seen in association with mycotic infections which have spread to or involve
the urinary tract.
Parasite ova are another infrequent sediment finding yet it’s important to be aware of their
existence and to be able to differentiate them from some of the common look-alike contaminants. The
ova of Capillaria, a tiny threadworm of dogs and other Canidae, may be found in urine sediment
although it’s questionable as to whether infection with Capillaria spp. Is associated with any welldefined pathology. Of greater significance are the eggs of the very rare parasite, Dictophyma Renale;
common name, “Giant Kidney Worm”.
Reporting of Results
Each laboratory should establish a standardized protocol for reporting out the results of urine
sediment evaluation. At least 10 microscopic fields should be observed for all elements and an
average taken with results being recorded in numbers/field. Cellular elements and bacteria are
reported in numbers/HPF (high power field) while casts and larger elements are best reported in
numbers/LPF. While this protocol can certainly be modified to meet the needs of each individual
laboratory, it is paramount that the reporting of results follow an established format each and every
time.
References and suggested Reading
Chew, Dennis J. and Steven P. DiBartola: Interpretation of Canine and Feline Urinalysis, Ralston Purina
Company, 1998.
Haber, Meryl H.: Urinary Sediment: A Textbook Atlas, ASCP Press 1991.
Osborne, Carl A. and Jerry B. Stevens: Handbook of Canine and Feline Urinalysis; Ralston Purina Company,
1981