Urinalysis - The Body Fluid of Choice for Disorders of the Urinary Tract and More

Animal technicians play an important role in the evaluation of urine. You are likely to be involved in collecting urine and then performing its analysis. Collection of urine without contamination (non-urinary chemicals or cells) and without trauma to the urinary tract (which introduces cells and protein into the urine) is critical to proper interpretation. The method by which urine is collected influences the cell and chemical content that will be reported, and should be clearly noted on the urinalysis form for reference. Urine may be collected by voiding, 'expression', catheterisation, or cystocentesis; each method has its own advantages and disadvantages. Results from urinalysis often provide diagnostic clues and therapeutic directions for patients with upper and lower urinary tract disorders as well as for those with endocrine and systemic diseases. Any time an animal is sick enough to submit blood samples, they are sick enough to submit urine samples. Urinalysis provides a view into body functions and processes that are often not apparent during assessment of blood analytes; pathological changes often occur in urine prior to changes in blood chemistry.

Voided Urine

When are voided urine specimens OK to submit? Midstream voided samples (urine collected after voiding has started) are acceptable for initial screening, but often urinalysis will need to be repeated on samples collected by another technique if abnormalities are disclosed from the voided sample. Voided samples are great for evaluation of urinary specific gravity, especially when collected first thing in the morning before the dog has eaten or drunk any water. This is the time that the urine is most likely to be concentrated - often greater than 1.040 urinary specific gravity (USG) in healthy dogs. Less than maximal urine concentration may provide clues to underlying renal and endocrine disorders. Voided samples are acceptable for evaluation of urinary pH if analysed within a few hours of collection. If blood is noted in a urine sample obtained by cystocentesis, a voided sample may be needed to determine whether the blood observed was caused by the cystocentesis needle. Voided samples are not acceptable for bacterial culture due to the potential for heavy bacterial contamination of the sample from the distal urethra and genital tracts.

Catheterised Urine

When are catheterised urine samples OK to submit? It is rarely justified to obtain routine urinalysis by catheter, since the possibility of introducing urinary-infection-causing bacteria is always a threat, particularly in female dogs. Some trauma to the urinary tract during catheterisation is unavoidable and can be expected to cause mild increases in red blood cells, transitional epithelial cells and protein in the urine. The initial 1–3 ml of urine from the catheter should be discarded (called a mid-stream catheterised sample), since the first few millilitres are most likely to be contaminated from the urethra and genital tracts. Results of urinalysis taken from animals with indwelling urinary catheters are more likely to have blood and protein present, secondary to the presence of the catheter.

Cystocentesis Samples

When are samples collected by cystocentesis OK to submit? In general, it is best to evaluate urine collected by cystocentesis (vesicopuncture), since this method bypasses potential contamination of the specimen with cells, protein or bacteria from the urethra, vagina, prepuce and perineum. This is unquestionably the method of choice for urine culture and microscopic evaluation of bacteria in sediment, since normal urine directly from the bladder should not contain any bacteria. In addition to the lack of contamination, this method is cost-effective compared to urethral catheterisation and poses no risk of creating urinary tract infections in the animal.

Cystocentesis should not be performed for routine urinalysis when it is likely that the bladder contains transitional cell carcinoma (seeding possibilities), in those with severe emphysematous cystitis, those with less than a 25 × 10⁹/l platelet count, and during recent severe caudal abdominal trauma. It is easy to introduce many red blood cells and protein into the urine sample that were not there prior to the puncture. Ultrasound guidance facilitates collection of urine samples via cystocentesis when the bladder is small or when the patient is large or obese. Cystocentesis should not be performed for routine urinalysis when the bladder is obstructed or if there is neurogenic bladder atony, as leakage of urine under pressure into the peritoneal cavity may occur.

Performing the Urinalysis

Urinalysis should be performed as quickly as possible following collection of the sample (within 15–30 minutes). Prolonged exposure of urine to room temperature before analysis can result in dissolution of delicate casts, change in pH, growth of bacterial contaminants and loss of cellular detail due to intracellular degeneration. Refrigeration of the specimen is necessary if examination within 15–30 minutes after collection is not possible. The diagnostic value of the urinalysis is greatly enhanced when the urine sample is obtained prior to initiation of diuretic or intravenous fluid therapy that may alter urine concentration.

A complete urinalysis involves measurement and reporting of physical properties, chemical reactions, and examination of microscopic urinary sediment. Performing a urinalysis without an examination of fresh urinary sediment is similar to running a complete blood count (CBC) without a differential count - in both instances lots of important information is never available for critical review. The same volume of urine should be centrifuged in all patients to allow semiquantitative comparison of any abnormal findings between animals or from the same animal over time.

Urinary specific gravity (USG) is the only overall indicator of renal function in the urinalysis and consequently is very important. Refractometers designed for analysis of human urine, but often used by veterinarians, have a scale from 1.001 to 1.035. One refractometer designed for veterinary use has a scale calibrated from 1.001 to 1.060 and is better suited for veterinary practice since USG often exceeds 1.035. It is no longer acceptable to report values as 'greater than 1.035' or 'off the scale', as potentially quantitative valuable information is lost about renal function. It should be noted that some refractometers manufactured in Japan use a different formula for analysis of USG than those made in other parts of the world - in our hospital, we have noted differences up to 0.010 between these instruments.

Dipstick reactions for urine chemistry are graded on a subjective scale from 0 to 4 plus, with 1 plus being a trace reaction and 4 plus being the most intense reaction possible. It is important that urine be at room temperature for dipstick testing, as some colour reactions are temperature-dependent. Urine should be well mixed prior to exposure to the dipstick to ensure that all constituents of the urine will contact the reagent pads. Colour reactions should be read in good light, as some of the reactions have subtle colour changes, particularly notable for protein content. Highly pigmented urine (obviously bloody or dark with bilirubin) can make it difficult or impossible to accurately determine the degree of colour reaction in some instances. Dipstick testing for white blood cells (WBC) is very unreliable in urine from dogs (many false-negatives) and cats (many false-positives). Similarly, dipstick testing should not be used to determine USG. Dipstick reactions for protein analysis can be falsely increased during the presence of highly alkaline urine following direct activation of the colour strip by the pH change. Highly concentrated urine may result in a positive dipstick reaction that is not of clinical consequence; dilute urine may have a negative reaction for protein despite the presence of clinically relevant proteinuria. Historically, it has been thought that dipstick protein measurements were reliable, but they are often falsely negative and falsely positive when compared to more sensitive and precise measurements of urinary protein (urinary protein to urinary creatinine ratio and microalbuminuria testing).

Evaluation of Urinary Sediment

The goal of centrifugation is to concentrate otherwise undetectable abnormal urinary elements for microscopic evaluation. A pellet at the bottom may or may not be macroscopically visible following centrifugation. Sedi-Stain® may be added to the sediment to enhance contrast of cellular elements; although this is optional, I do recommend it. The microscopic slide is first examined under low power to count casts and to detect areas of interest that need examination under high power. At least 10 high-dry microscopic fields are then evaluated to quantitate white blood cells, red blood cells, epithelial cells and bacteria, and to examine crystals that might be present. Casts are counted per low-dry power field. It is a good idea to bias the examination to include the coverslip margins, as elements often accumulate there.

Urinary sediment from healthy animals contains very few cells or casts and no bacteria, but can contain certain crystals. The technician's ability to properly identify red blood cells, white blood cells and bacteria is most important. Do not expect cells in urine to look like they do on a blood film due to the widely varying effects of urinary osmolarity on the cells as well as that from urinary pH and urinary toxins. The presence of up to five red and five white blood cells per high-dry microscopic field is considered normal when the sample is obtained atraumatically by catheterisation or cystocentesis. Slightly higher numbers of cells (up to eight red or white cells) may still be considered normal when a voided sample is examined. Large numbers of red but not white cells easily enter the urine during cystocentesis at times despite excellent mechanical technique of the sampling. The presence of clumps of white blood cells increases the probability that an organism is the cause of pyuria, and clumps should be so noted on the form.

Epithelial Cells

In health, only occasional transitional epithelial cells should be present in urine. Transitional epithelial cells can enter the urine as a result of inflammation in the renal pelvis, ureter, bladder or proximal urethra. Clumps of transitional epithelial cells can be observed after severe urinary tract inflammation, and can occur in animals with neoplasia. They are rarely encountered in cats with idiopathic cystitis. A dry mount cytological preparation of urine should be examined for morphology of these epithelial cells if rafts are consistently identified in the urinary sediment. Squamous epithelial cells can be observed in voided specimens as contaminants with no particular clinical relevance.

Transitional epithelial cells vary widely in size, and are usually rounded, but only small ones (approximately 1.5–2 times the size of white cells) are derived from the kidney. Unfortunately, small transitional epithelial cells can also originate from the lower urinary tract. There is no way to definitively prove the origin of small free epithelial cells in the urine unless they are within a cast. Small transitional epithelial cells with a tail-like configuration (caudate cells) are thought to arise from the renal pelvis and consequently their presence may suggest upper urinary tract localisation of disease. The presence of sheets or clumps (rafts) of transitional epithelial cells strongly suggests neoplasia, but may also occur with severe inflammation. A dry mount cytological preparation of urine should be examined for morphology of these epithelial cells if rafts are consistently identified in the urinary sediment.

Bacteria

When urine samples from healthy animals are properly collected and examined in a timely manner, no or very few bacteria should be seen. Particles of debris and very tiny crystals may look like cocci when subjected to Brownian motion in urine sediment, and the laboratory may report a false-positive for bacteria. It is easier to be confident that bacteria are present when rod-shaped organisms are seen. Specimens that are reported positive for bacteria should be Gram stained for confirmation, and a quantitative urine culture should be performed. The absence of light microscopically visible bacteria does not ensure that bacteria are absent; at least 10,000 rods/ml or 100,000 cocci/ml of urine must be present to be visible microscopically.

Casts

Casts are moulds of proteins and cells that form within the lumen of the distal tubule and should be encountered only rarely in urine from healthy animals. Cellular casts in urine are always considered pathological regardless of their quantity.

Cellular casts are easily disrupted and can undergo rapid cellular degeneration, so it is essential to examine fresh urinary sediment if cellular casts are to be identified. The presence of cellular casts localises a pathological process to the kidneys. Cellular casts may consist of red blood cells, white blood cells or renal tubular epithelial cells. Red blood cell casts are occasionally observed in acute glomerulitis and following severe renal trauma or renal biopsy. White blood cell casts (pus casts) are indicative of renal inflammation and are often thought to be caused by bacterial infection. Epithelial cell casts result as the lining of the renal tubule sloughs following a variety of injuries to the kidney - indicating severe tubular injury. Red blood cell (RBC) casts indicate some type of renal haemorrhage, often that from acute glomerular bleeding. Acute glomerular disease is not common in domestic animals.

It is easy to identify the type of cellular cast when the morphology of the cells within the cast is well preserved. When cellular degeneration has occurred it can be difficult to tell the difference between white blood cell and epithelial cell casts. Where cell type cannot be accurately determined, the cast is referred to as a degenerating cellular cast. Since even a single cellular cast is of great diagnostic significance, it is important to note their presence. Cellular casts are especially fragile and their presence easily missed if urine is stored too long prior to examination.

Granular casts are more commonly encountered than cellular casts in animals with renal disease. According to the classic theory of Addis (Figure 1), granular casts develop from degenerating renal epithelial cells, white cells and red cells that have remained within the renal tubular lumen. Granules can also originate from precipitation of filtered serum proteins into tubular fluid.

Waxy casts consequently require the longest intrarenal time for their development. Waxy casts are translucent and sometimes take up stain intensely; they represent the final transformation of cellular breakdown within the renal tubules. They tend to be brittle, often with visible fractures and sharp, broken-off ends. They are not fragile casts, and are stable for some time in alkaline or acid urine. Since it takes more intrarenal time to form this cast, their presence implies local nephron obstruction and often indicates advanced renal disease.

Hyaline casts are pure precipitates of matrix (Tamm-Horsfall) mucoprotein (THP). Hyaline casts are transparent and have low optical density. They can be missed during brightfield microscopy if lighting intensity is not reduced. The presence of persistent hyaline casts usually indicates increased filtration of serum proteins which does not happen in health. Increased filtered proteins can occur from glomerular disease, passive congestion and fever. Increased concentration of THP favours its precipitation - this can occur in highly concentrated urine and from increased tubular secretion. Decreased tubular flow rate and the presence of myoglobin or haemoglobin in the tubular fluid favour precipitation of THP.

A 'broad cast' refers to a very large cast, based on its width. It can only form in the collecting tubule or in some pathologically dilated portion of the distal nephron. The implication is that there is very slow tubular flow in order for a cast of this size to form.

Figure 1. Addis theory of cast formation. According to this theory, granular casts result from the breakdown of cells and waxy casts from additional degeneration of granular casts. Casts on the far left show cellular casts formed by the aggregation of RBCs, WBCs or renal tubular epithelial cells. Degenerating cellular casts are those in which the cell type of origin cannot be definitively identified. Coarsely granular and then finely granular casts represent further degradation of the degenerating cellular cast. The waxy cast is the final transformation of a granular cast to a translucent (not transparent) protein.

Crystals

The presence of crystals in urine is often more confusing than helpful in providing meaningful information. Many amorphous crystals cannot be definitively identified based on morphology alone. Urinary pH can suggest which types of crystals are more likely to precipitate out of solution at a particular pH. Crystals can be identified in those without stones, in those with stones and sometimes in those with stones of another crystal composition, so their clinical significance is questionable in many instances. It is very important to remember that crystals can come out of solution after collection of the sample, especially during storage and even more so during refrigeration. Crystals that are reported may not have been there at the time the sample was collected.

Struvite crystals are common in both normal and abnormal small animals and do not mean that the animal has urolithiasis due to struvite. Struvite crystals are the most common type encountered in small animals. Struvite is easily identified when they assume the 'coffin-lid' appearance but they can also assume amorphous forms. They form more often in alkaline urine.

Calcium oxalate crystals can be helpful in establishing a diagnosis of ethylene glycol (antifreeze) poisoning in the appropriate clinical setting, but they can also be seen in the urine of healthy animals. There are many different morphological appearances for calcium oxalate crystalluria. These crystals are more often found in acid urine. The dihydrate form of calcium oxalate is relatively easy to recognise due to its rhomboid shape with internal Maltese cross pattern. Oxalate crystals may be an artefact of storage and refrigeration or may be associated with urolithiasis, hypercalcaemia or ethylene glycol ingestion.