A great deal can be achieved in the practice setting when it comes to urine analysis, and the high proportion of samples analysed in house means that there is generally more expertise and familiarity than with some other aspects of clinical pathology. Equipment requirements are basic and cheap.
Sample Collection
Ideally samples should be harvested by cystocentesis, especially if there is a necessity for culture, but catheterised samples may also be used. For the sake of practicality and patient safety, free-catch samples are most often employed; contact with the distal urinary tract, fur and non-sterile receptacle make these samples less helpful for culture, but sediment and dipstick results are still valuable.
Gross Examination
The colour, turbidity and odour of the sample should be recorded. Haemorrhage and haemoglobin lend a pink tinge, at relatively low values, myoglobin appearing more brown; unfortunately there is enough overlap between the pigments not to be able to distinguish on visual inspection. Samples with a heavy crystal or cell content (such as those from rabbits) may settle into distinct bands if allowed to stand.
Specific Gravity
This should be measured by refractometer, not by reagent pads on dipsticks.
Quality control for refractometers includes measuring the specific gravity (SG) of distilled water (should be 0.0), and 5% NaCl (should be 1.022). Using two measures allows a check at low and mid ranges.
SG depends on particles in urine: number, size and weight. This becomes important so that, for example, diabetic animals with glucosuria are not incorrectly interpreted as having a higher SG due to dehydration. SG needs to be measured at room temperature to be consistent.
Point readings of urine SG of >1.030 in the dog and >1.035 in the cat give a presumption of concentrating ability.
Urine Dipsticks
Common problems include utilising panels that are not designed for canine and feline patients, or correlate poorly with gold standard methods. Leucocyte and SG panels are not particularly accurate and should be avoided. pH is more accurately measured with a pH stick, which is available from scientific suppliers; the benefits of accuracy carry over into the interpretation of crystals in the sediment. Pigment in the urine can render the panels impossible to read, and having alternative methods to hand (sediment examination, pH stick) can be valuable.
Quality assurance is available for urine dipsticks in the form of 'checkstix', allowing a degree of certainty about the performance of a particular batch.
Urine Sediment
Many factors influence the findings in urine sediment, beginning with the method of collection, which may greatly influence the findings expected. Free-catch samples and some from catheterised animals may contain squamous epithelial cells from the external skin, genitalia or distal urethra. Lubricating gel appears as a curiously spiky crystalline material under high magnification which may be mistaken for amorphous crystals, whereas starch granules from glove powder gives a clear crystal with a central point that may also be difficult to identify.
Parasite eggs, vegetable material, fungi and bacteria are all potential contaminants in samples collected from the floor, and this method should be avoided.
The collected urine should be placed into a plain tube for sediment examination: the addition of boric acid to samples for sediment examination will change the pH and other findings. Boric acid should be reserved for postal samples that will be cultured, as its purpose is to prevent overgrowth. There are many different colour tops for sterile universal tubes and the manufacturers have not standardised these, so double check.
Once the sample is collected, prompt examination will prevent deterioration and bacterial overgrowth; if the sample can not be examined immediately, then it should be refrigerated and allowed to come back to room temperature before examination.
For sediment examination, it is important to standardise the volumes used as far as possible to allow comparison between samples. Ideally, a volume as large as 5-15 ml should be spun at 1500 rpm for 5 minutes to give a sediment pellet. In practice, such capacious volumes are not always possible (especially in cats and given that many centrifuges will not hold test tubes of this size), so smaller volumes may be used as long as this becomes a standard for that practice. The supernatant is then pipetted off, leaving approximately 0.5 ml that is flicked gently to resuspend. Stains may be added at this point (again, using the same amount each time and factoring in the dilution into counts), or the sediment examined with background illumination alone.
To ensure equal volumes are examined, standard graticule grids which contain a precise volume of urine and have an easy-to-focus-on grid for counting can be used. Care should be taken not to allow overspill from one grid to another. Alternatively, pipetting just enough urine onto a slide for a 22 mm2 coverslip to spread the sample with no overspill gives a rough standardisation.
The slide should be examined at x100 magnification (x10 lens) for the quantification of casts and at x400 (x40 lens) for the identification and quantification of cells and crystals. Often a semiquantative approach is used (scant to ++++) to take account of the variations in volumes used, but more properly items in the sediment should be averaged to give a number per power field.
Cellular Elements
Epithelial cells may be difficult to see detail in using a sediment stain alone, and this is certainly not adequate for nuclear features: sediment should be dried and stained with a cytological stain if the question is 'Has this animal a transitional cell carcinoma?'
Squamous epithelial cells are generally recognisable because of their large size and folded, angular appearance. Transitional cells are medium sized, roundish cells, often larger than their cuboidal renal counterparts. Size and shape of cells should be consistent in any adherent rafts of cells, and while low numbers are a normal finding, high numbers in a sample collected atraumatically would raise suspicions of inflammation or neoplasia.
Leucocytes may also be picked out on sediment, although with lack of nuclear detail they may be surprisingly hard to distinguish from transitional epithelial cells. Leucocytes should show no cell-to-cell adherence, however.
Leucocyte and erythrocyte numbers vary a little due to collection method (some small amount of blood may be expected in a cytocentesis sample), but generally around 0-5 cells per high power (x400) field is acceptable.
Casts
Casts are formed from secreted Tamm-Horsfall protein in the tubule, cells, or a mixture of the two. In health the amount of Tamm-Horsfall protein is insufficient to read as a positive result on urine dipsticks. Where there is a process that slows transit of urine through the tubules, the number and nature of casts may alter as this allows a more formed cylindrical cast to be made and any cells, dying cells or cell breakdown products to be impressed into it. Generally low numbers of casts may be disregarded, although waxy casts are always regarded with suspicion. What the cast does is allow a snapshot of events occurring high in the tubule--thus, red cell casts correspond with renal haematuria rather than haemorrhage from cystitis, and white cell casts with pyelonephritis.
Crystals
Crystals in urine sediment are a frequent finding, and are not necessarily pathological. Their formation is dependent on numerous factors including pH, solute type and concentration and temperature. Further factors such as the presence of a nidus (e.g., bacteriuria) may influence the formation of calculi, although calculus type and the crystals present in the urine on a point sample may not match.
Crystals may form a recognisable geometric habit or may be more amorphous; identification in the latter case is more difficult and may depend on proving solubility at different pHs.
Common crystals such as struvite may be an incidental finding in healthy carnivores, and over-interpretation should be avoided. Calcium carbonates are expected in lagomorphs. In other instances, even low numbers of a certain crystal type should be concerning--calcium oxalate monohydrate crystals may be associated with ethylene glycol poisoning, and should prompt a rapid investigation of this possibility. Other crystals have strong correlations with medical conditions, e.g., ammonium biurate is associated with poor liver function/portosystemic shunting.
Other Findings
As well as cells, crystals and casts, organisms, lipid droplets and spermatozoa are commonly identified in sediment. The presence of bacteriuria with inflammation is consistent with urinary tract infection assuming a sterile collection technique. Bacteria without inflammation are commonly seen as contaminants and less frequently in immunosuppressed animals, hyperadrenocorticism being the most commonly encountered cause.
Summary
In summary, there is a great deal of information available from urine analysis in practice with minimal equipment. Careful sample collection and efforts to standardise techniques give the best results.