Section of Clinical Pathology, Faculty of Veterinary Science, University of Pretoria
Onderstepoort, Republic of South Africa
Urinalysis is essentially a standard component of the clinical minimum database in many veterinary teaching hospitals and referral practices. Many general practices also consider a urinalysis as essential. It, therefore, begs the question, what is it that one gets out of such an investigation that makes it so important?
Urinalysis in human medicine
For urinalysis to be considered essential it would have to be a test (set of tests) with a high data yield or high sensitivity. In general, neither the medical nor the veterinary literature supports such a view, and consequently its role in the MDB should be questioned and revisited. The basic question as to whether urinalysis should be performed, as a "screening" test, on all patients, has received considerable attention in the medical literature. The weight of the evidence suggests that this is a waste of resources and that clinical decisions are only altered in a very small percentage of cases, most of which would have been identified as requiring urinalysis on other grounds.
Urinalysis in veterinary medicine
There have been no convincing, large clinical studies on this issue in veterinary medicine. The little that has been published suggests that there are important extenuating circumstances that excludes veterinary urinalysis from this argument.
Several publications have shown that the urine SG patch, available on some dipsticks, is quite unsuitable for canine urine SG estimation (and probably for feline urine as well).
In veterinary medicine a case can be made for the retention of urine sediment examination as a potentially important diagnostic procedure on the following grounds AND provided a reasonably time-economical approach can be used.
Urine sediment cytology for pyuria is a contentious issue in human medicine. The medical literature provides overwhelming evidence that sediment microscopy for pyuria is of very limited use and can effectively be replaced by modern dipsticks that test for nitrite (for bacteriuria) and neutrophils (leukocyte esterase test for leukocyturia) patches appear to be fairly reliable. Some very thorough investigations, on both canine and feline urine samples, showed that neither method was sufficiently reliable in those species. Consequently, until an alternate methodology becomes available, identification of bacteriuria and leukocyturia, in small animal urinalysis, will have to be based on microscopic examination. Consequently, as it must be retained in veterinary work anyway, the added advantage of combining this with a search for other diagnostically useful elements such as renal tubular cells and casts, sensitive indicators of acute renal pathology, can enhance its usefulness.
An additional bonus is that the blood patch's lack of specificity is resolved if microscopy is going to be conducted anyway.
Urine sediment cytology--a rational approach
Whether the sediment should be stained appears to be a matter of personal preference. The author, based on twenty-five years experience, recommends Sternheimer-Malbin (Sedistain) supra-vital staining. The single biggest problem with urine sediment examination is that the average urine sample produces a large amount of essentially unrecognizable material that precipitates and is often stained by the stain. In order to get around this problem one has to follow a rather "draconian" algorithm of what to search for and what to ignore. Unless this is done, urine sediment examination becomes an exercise in futility with an inordinate amount of time spent on making "calls" on highly dubious identification decisions. There is no rational way in which one can charge for the time spent and eventually the veterinarian abandons urine sediment examination as a nightmare that need not be relived.
Preparing the sediment
Place a convenient (often limited by what is available) volume (ideally approximately 5 ml) in a centrifuge tube and spin it down at about 4 000 to 5000 rpm (in a standard bench centrifuge) for approximately 10 min. The intention is to get everything that is not in solution, and is worth looking at, into the centrifuged pellet. There is no cell or structure that is going to be significantly altered (to the point of being unrecognizable) by fast/hard centrifugation in a bench centrifuge.
Pour off the supernatant (preferably into another tube so that SG and dipstick chemistries can be conducted) by inversion, allowing the centrifuged tube to drain for a few seconds.
When the tube with sediment is held vertically again, there will be sufficient residual urine coating the tube, that will run down onto the pellet, to allow the pellet to be suspended by gentle tapping of the bottom of the tube.
Now, add a SMALL drop of Sternheimer-Malbin stain, and gently tap the re-suspended pellet again.
Using a Pasteur pipette (or similar device), place the tip into the stained sediment and allow capillary action to cause it to rise to about half a cm. There is no need to apply negative pressure to the pipette.
Remove the pipette and the column of stained sediment will remain in the pipette by capillary forces (again, no need to use a rubber teat or closing device).
Touch the pipette tip to a slide glass (and if necessary, place a finger over the wide, open end and exert a little pressure), allowing a drop to emerge.
Carefully place cover-slip on the drop without any lateral movement.
The algorithm is as follows
Step 1. Examine the coverslipped, stained sediment under a microscope using a 10 X ocular and 10 X objective lens, making sure that the condenser is lowered to create a little contrast.
Step 2. Look for BIG things. The only things that are big and diagnostically useful, are bladder epithelium cells and casts. Squamous epithelium cells, although of no significance, must not be confused with bladder cells. Anything else that is BIG and is not one of the above, does NOT EXIST.
Step 3. Engage the 40 X objective lens, raise the condenser a little and look for things that are SMALL. The only SMALL things that are worth recognising are: leukocytes, erythrocytes and "bugs" (bacteria and yeasts etc). All leukocytes are neutrophils until proven otherwise by someone else at some other time (i.e., DO NOT try to conduct a leukocyte differential count-it is not worth the time and effort-you may miss one lymphoid sediment in a lifetime but the sacrifice is well worth it). All blue-black and dark blue granular material is a pigment of your imagination and does not exist.
Step 4. Go back to 10 X ocular and 10 X objective lens, making sure that the condenser is lowered to create a little contrast and now look for things that are INTERMEDIATE in size. If you do this under 10 X 40 you tend to "get lost". There are only really two things that are important to recognize in this size group: Renal Tubular Epithelial (RTE) cells and urethral epithelium. The latter is virtually only ever found in urine from cases that were catheterized and provides very little diagnostic information. Consequently, the only INTERMEDIATE sized thing in the sediment that you need to concern yourself with is the RTE cell. This cell is very intensely maroon-red and very granular and difficult to confuse with anything else.
Ignore everything else, regardless of how tempting and entertaining it may seem. The only possible exception to this rule is the rare, but diagnostically useful, ammonium biurate crystals and the really rare cysteine crystals. All other crystals are there to tease you.
This approach is, in fact, very difficult to do the first few times because there seems to exist an innate urge in veterinarians to identify everything they see. The "trick" is to ignore as many things as possible.