What is a urinalysis?
Urinalysis is a routine test that reports the physical and chemical properties of urine. It is used mainly to assess the health of the kidneys and urinary system, but it can also reveal problems in other organ systems, and is important for diagnosing metabolic disease such as diabetes mellitus. It is a valuable test in both healthy and sick animals and should be included in any comprehensive evaluation of a pet's health.
How is urine collected?
There are three main ways to collect urine in cats and dogs.
Cystocentesis: A sterile needle and syringe are used to collect urine from the bladder. The needle is passed through the abdominal wall into a full bladder and urine is withdrawn directly into the sterile syringe. The advantage of cystocentesis is that the urine is not contaminated by miscellaneous debris from the lower urinary passage. This type of sample is ideal for assessing the bladder and kidneys and for detecting bacterial infection. The disadvantages of cystocentesis are that the method is slightly more invasive than other methods and it is difficult to do in patients that are uncooperative.
Catheterization: A very narrow sterile catheter is passed up the lower urinary passage (called the urethra) into the bladder. A sterile syringe is attached to the catheter and urine is withdrawn from the bladder into the syringe. The technique is less invasive than cystocentesis and is a good option when a voluntary sample is not available, especially in a male dog. Catheterization causes mild irritation to the urethra, and may carry bacteria from the urethra into the bladder.
Mid-stream free-flow: Urine is voided voluntarily by pet in the usual way and a sample is collected into a sterile container as the pet urinates. Ideally, the sample is collected mid-stream, meaning partway through urination. This type of sample is often called a “free-flow” or “free catch” sample. The advantages of this method are that it is completely non-invasive, and the pet owner can collect the urine sample at home. The disadvantages are that it may be difficult to collect a sample in mid-stream from some pets, and the urine is more likely to be contaminated by miscellaneous debris from the urethra or the environment.
How is a urinalysis performed?
There are four parts to a urinalysis.
- Assess appearance: color and turbidity (cloudiness).
- Measure concentration (‘density’) of the urine.
- Measure pH (acidity) and analyse chemical composition of the urine.
- Examine the cells and solid material present in the urine using a microscope.
Most of the analysis is done on whole urine (as it comes from the animal), but the microscopic examination of cells and solid material requires the sample to be concentrated or sedimented. To do this, urine is placed in a tube and then centrifuged (spun in a circle at very high speed) to force the cells and solid material to settle to the bottom. This accumulated material, or sediment, is collected and spread on a slide, and then examined under a microscope.
What do changes in color and turbidity (cloudiness) mean?
Normal urine is pale yellow to light amber, and is generally clear to slightly cloudy. Urine that is dark yellow usually suggests the pet needs a drink of water or may be dehydrated. Urine that is very pale yellow or clear suggests the pet is drinking a lot of water and urinating frequently; this may signal underlying kidney disease, or a disorder that interferes with the pet’s ability to pass concentrated urine. Urine that is any color other than yellow (for example orange, red, or brown-black) may contain substances not normally found in healthy urine and may reflect injury or underlying disease.
Increased turbidity or cloudiness indicates that there are cells or other solid material in the urine. Examination of the sediment will determine what is present and whether it is significant. Increased turbidity is typically associated with the presence of blood, inflammatory cells, crystals, mucus, or debris.
What is specific gravity and how does it help detect disease?
It may help to think of urine specific gravity as the density of the urine. A healthy kidney should produce dense (concentrated) urine, while watery (dilute) urine may signal underlying disease.
One of the kidney's jobs is to maintain the body's water level within relatively narrow limits. If there is an excess of water in the body, then the kidneys allow the excess water to pass out in the urine, and the urine becomes more watery or dilute. If there is a shortage of water in the body (as in dehydration), then the kidneys reduce the amount of water lost in urine, and the pet passes more concentrated urine.
Normal animals may pass dilute urine from time to time during the day, and a single dilute urine sample is not necessarily a cause for concern.If a pet continues to pass dilute urine, then there could be underlying kidney or metabolic disease and further investigation is recommended.
What is urine pH and why is it measured?
Urine pH is a measure of how acidic or alkaline the urine is. The pH can change with diet, but can also signal the presence of infection or metabolic disease. Normal urine in the cat and dog ranges from mildly acidic to mildly alkaline. Extremes in urine pH beyond this range are more likely to be associated with disease.
How is the chemical analysis of the urine performed?
The chemical analysis of urine is performed using a dipstick, which is a small strip of plastic that holds a series of individual test pads. Each test pad measures a different chemical component and changes color to indicate the amount of that substance in the urine. The dipstick is dipped into the urine, and after a short waiting period, the color of the test pads is compared to a chart that translates the intensity of the color to an actual measurement.
What substances are detected by the chemical analysis of urine?
The presence of protein in urine is called proteinuria. Mild proteinuria in a concentrated urine may not be cause for concern, but proteinuria in dilute urine should be investigated since it may signal developing kidney disease. The significance of proteinuria is often determined by doing a second test called the protein:creatinine ratio (see handouts "Urine Protein", and "Urine Protein:Creatinine Ratio").
Glucose should not be present in the urine of healthy cats and dogs. The presence of large amounts of glucose usually indicates the pet has diabetes mellitus. Small amounts of glucose in the urine may also be found in pets with kidney disease.
Ketones appear in urine whenever the body breaks down excessive amounts of stored fat to meet its energy needs. This occurs most frequently in diabetes mellitus, but can also be found in healthy animals during prolonged fasting or starvation.
Blood in the urine usually indicates there is bleeding somewhere in the urinary system. Sometimes this is due to how the sample was collected; for example, small amounts of blood are often found in samples collected by cystocentesis or catheterization. Blood in the urine is associated with diseases such as bacterial infection, bladder stones, trauma, or cancer, so if blood in the urine does not appear to be due to the sampling method, further investigation is recommended.
A positive reading for blood can also be seen with a disease called hemolytic anemia, in which red blood cells are destroyed and a protein called hemoglobinis released. Hemoglobin passes into the urine and causes the blood test pad to show positive, even though there is no actual bleeding in the urinary system.
Occasionally the blood test pad will show positive for blood when there is muscle inflammation or injury. This is because damaged muscle fibres release a protein called myoglobin, which is very similar to hemoglobin. Myoglobin will also cause the blood test pad to show positive, even though there is no actual bleeding in the urinary system. A specific test for myoglobin can be done if muscle injury is suspected.
The presence of urobilinogen in urine indicates that the bile duct is open and that bile can flow from the gall bladder into the intestine. A negative urobilinogen result has no interpretation and does not mean the bile duct is obstructed.
Bilirubin is a substance that is produced in the liver and normally excreted in the bile. Bilirubin is not found in the urine of healthy cats but may be found in small quantities in the urine of healthy dogs. Abnormal amounts of bilirubin in the urine are associated with liver disease or red blood cell destruction (“hemolysis”), and should always be investigated.
Why examine the urine sediment?
Urine sediment is the material that "sediments" out or settles into the bottom of the tube when a urine sample is spun in a centrifuge.
What sorts of things can be found in a urinary sediment and what do they mean?
The most common things found in urine sediment are red blood cells, white blood cells, crystals, bacteria, and tissue cells from different parts of the urinary system. Small amounts of mucus and miscellaneous debris are often found in free-catch samples. Rarely, parasite eggs are found in urine.
Red Blood Cells. Small numbers of red blood cells are often found in urine collected by cystocentesis or catheterization, but large numbers of red blood cells usually indicate bleeding. This may be caused by conditions such as bladder stones, infection, coagulation problems, trauma, cancer, etc.
White Blood Cells. Small numbers of white blood cells in a free-catch sample may not be significant, but in general, an increased number of white blood cells indicates inflammation somewhere in the urinary system. Inflammation is often secondary to bacterial infection.
Bacteria. The presence of both bacteria and inflammatory cells in the sediment indicates there is likely bacterial infection somewhere in the urinary system. Ideally, the urine should be sent to the laboratory for culture and sensitivity testing to find out what types bacteria are present and which antibiotic should be used to treat the infection.
Crystals. There are many different types of crystals and they vary in size, shape, and color. The significance of crystals also varies. Some crystals are unique and help to pinpoint a specific diagnosis. In more common conditions such as bladder infection and bladder stones, the crystals provide information that can influence how the disease is managed.
Crystals in the urine do not always indicate disease. Some crystals form when a pet is given certain types of medications. Crystals can also form in urine after it has been collected, especially if there is a long delay before the urinalysis is done. If this happens, the veterinarian may wish to examine a fresh sample immediately after it has been collected to determine if the crystals are significant.
Image via Wikimedia Commons/Joel Mills (CC BY-SA 3.0.)
Tissue Cells. Increased numbers of tissue cells are often seen in samples collected by catheterization. While this is not a sign of disease, increased cellularity can be seen with a variety of disorders, including urinary tract inflammation, bladder stones, prostate problems (in the male dog), cancer etc. If the cells look abnormal, your veterinarian may recommend a cytological preparation of the sediment, which allows for a more detailed examination of the tissue cells.
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