In previous newsletters, we discussed the interpretation of complete blood counts and blood chemistry. The final component of routine clinical pathology is urinalysis. Kidneys can be thought of as factories responsible for: a) removing waste products and drugs from the body; b) balancing body fluids; and c) performing other essential metabolic functions. Where do the products from all this hard work go? The urine, of course! For this reason, examination of urine samples gives us essential information about the work kidneys are doing and how well they are doing it.
Urinalysis is the physical, chemical and microscopic evaluation of urine. Let’s explore routinely performed tests and what their results mean.
Color: Normal urine is transparent, and yellow or amber on visual inspection. Abnormal urine color may be caused by presence of pigments or blood, but it does not provide specific information about abnormalities by itself.
Clarity: Urine is normally clear. When pigment, infection, crystals, or fat are present, urine can become cloudy.
Specific Gravity: This test is a measure of urine concentration. Elevated urine specific gravity indicates dehydration, and that the kidneys are reabsorbing as much fluid as possible. Low specific gravity indicates the urine is diluted. Normally, this means the kidneys are excreting extra water due to, for instance, excessive water intake or administration of intravenous fluids. However, dilute urine in a clinically dehydrated animal is abnormal and could be caused by disease states such as kidney failure, hypoadrenocorticism (Addison’s disease), hyperadrenocorticism (Cushing’s disease), high blood calcium, or diabetes mellitus.
Urine pH: Urine normally is acidic in dogs but varies depending on diet, medication administration, and/or presence of disease. For example, some bacterial urinary tract infections can result in alkaline urine. Urine pH affects the formation of crystals and stones in the bladder and kidneys. Some types of kidney or bladder stones can be treated or prevented by feeding diets that manage urinary pH.
Protein: As a general rule, normal urine should not contain protein. Protein in urine (proteinuria) is a result of numerous causes including strenuous exercise, kidney disease, and infection. Because so many different conditions cause proteinuria, the condition must be interpreted in light of many other factors. For example, a trace amount of protein in concentrated urine is less significant than a trace amount of protein in dilute urine. If proteinuria is present without an obvious explanation (e.g., dehydration or evidence of infection), its significance can be verified by measuring the urine protein concentration in the context of kidney function, indicated by the urine creatinine concentration. This test is called a urinary protein creatinine (UPC) ratio and is not part of a routine urinalysis.
Glucose: Normally, glucose should not be present in urine. By far, the most common reason for glucosuria is diabetes mellitus. Less common causes of glucosuria include serum glucose overload, such as from intravenous administration, or damage to the kidney tubules resulting in excretion of glucose into the urine.
Ketones: Normally there should not be ketones in the urine, as ketones are abnormal compounds produced when the body burns fat instead of carbohydrate. The presence of urinary ketones usually is associated with diabetes mellitus, but can sometimes be seen with consumption of a low-carbohydrate diet, prolonged fasting or starvation.
Bilirubin/Urobilinogen: Bilirubin is a byproduct of red blood cell breakdown and is excreted by the liver. In dogs with concentrated urine, a small amount of urinary bilirubin can be normal, especially in male dogs. However, high urinary bilirubin often is associated with liver disease or diseases causing accelerated breakdown of red blood cells. A small amount of urinary urobilinogen also is normal, however the test is not specific enough to be clinically useful.
Occult Blood: A positive result on this test for blood in the urine can be due to hemorrhage, accelerated breakdown of red blood cells, or the presence of a byproduct of severe muscle breakdown called myoglobin. Discolored urine may yield false-positive results. A positive result should be followed with microscopic examination of urine sediment for red blood cells.
The next series of tests are performed microscopically on urine that has been centrifuged to allow heavier particulates and cells to fall out (the sediment). The sediment is collected and examined microscopically.
Red Blood Cells: Normal urine should contain very few red blood cells. Increased red blood cells in urine samples indicates bleeding somewhere in the urinary or genital tracts.
White Blood Cells: Normal urine should contain very few white blood cells. Increased white blood cells (pyuria) can occur due to inflammation, infection, trauma, or cancer.
Epithelial Cells: Transitional epithelial cells, shed into the urine from the bladder and proximal urethra, are sometimes seen in urine sediment. In a voided urine sample, squamous epithelial cells may be seen. However, cancerous transitional cells or other abnormal cells may be seen in an animal with certain types of bladder cancer.
Casts: Casts are clumps of cells. A few casts found in urine are considered normal. However, presence of casts in high numbers indicates kidney damage, and may be one of the earliest laboratory abnormalities noted with toxic damage to renal epithelial cells.
Infectious Organisms: The kidneys and bladder are sterile so the presence of bacteria in urine collected by cystocentesis usually indicates infection. Small numbers of bacteria from the skin of the genitalia may contaminate voided samples or samples collected by catheterization and do not indicate infection.
Crystals: Many different types of crystals may be present in urine and the type of crystal formed depends on urine pH, urine temperature, and length of time between urine collection and examination. Crystals in urine are not necessarily pathologic, but some types of crystals may indicate illness or an inherited metabolic problem. Struvite crystals are commonly seen in dogs and are not generally a problem unless there is a concurrent bacterial urinary tract infection. Calcium oxalate crystals are less common in dogs and, if persistent, may indicate an increased risk of calcium oxalate bladder stone formation. Rarer crystal types include ammonium acid urate crystals, which suggest liver disease, and an unusual form of calcium oxalate crystals, seen in association with antifreeze toxicity.
Lipids: Fat droplets commonly are present in urine from dogs and are not considered to be pathologic.
Spermatozoa: Spermatozoa may be seen normally in urine collected from reproductively intact male dogs.
Next time you are in the yard, with your collection tool of choice in hand to capture your dog’s urine, keep in mind all the valuable information the Golden Retriever Lifetime Study is gaining from your efforts.