At The Laboratory With My Pet’s Doctor: Urinalysis
In an ongoing series, MyPetsDoctor.com has informed you about laboratory tests your pet is likely to have performed as routine screening tests, as preoperative tests, and as tests designed to diagnose illness.
Today we’re going to look at tests commonly performed on urine.
The most basic is urinalysis, which is performed in two parts. First, let’s start with the chemical parts of the urinalysis. These are usually performed with a dipstick, shown at left, so-called because it is dipped into a urine sample to begin the testing process. A dipstick is a strip of plastic with paper squares attached, onto which have been added certain chemicals which can detect components in the urine both normal and abnormal. The amount of each urine component is indicated by the amount of color change on the square. The color on the strip is compared to colors on the chart to determine the test result.
Let’s start at the top of the dipstick and work down.
pH is the first test square. The pH: of the urine tells us how much acid is present or absent. Most dogs’ and cats’ urine should be a pH of 7.0 (neutral) or less (meaning more acid). A high pH may indicate a recent high-carbohydrate meal, an abnormality in metabolism that is causing the urine (and possibly even the blood) to have a pH that’s too high, and in some cases can indicate urinary tract infection caused by bacteria that are splitting nitrogen molecules, thus creating base, the opposite of acid. So far, we’ve looked at only one square, and we already have a lot of information.
Color change in the next test square indicates the presence of blood in the urine. The presence of blood in the urine is always abnormal. If there is enough blood in your pet’s urine that you can see red with the naked eye, you should call your pet’s doctor right away. A positive test for blood may result from infection, physical trauma to the urinary tract (such as a blow from a car or a blunt object), bleeding growths or stones in the bladder. Sometimes tiny blood vessels are damaged in the process of obtaining the urine sample. If the amount of blood is very small, the clinician may take that possibility into account.
A test square to indicate protein in the urine is very important, because several kinds of urinary tract disease “spill” protein into the urine. Infections in the bladder may damage blood vessels, releasing protein from the blood, infections and other diseases may damage the kidneys themselves, causing loss of protein. Anything more than the tiniest amount of protein in the urine is considered abnormal. Some inherited diseases of the kidneys that occur in certain breeds of dogs and cats will first be detected in the very young animal by testing the urine for protein.
Never should there be glucose (sugar) in the urine of a normal patient. Under certain circumstances a diabetic patient may be allowed to have glucose in the urine temporarily, but even then not for long periods of time. The presence of glucose in the urine may mean kidney damage is allowing glucose to be “spilled” into the urine. It can also mean that blood levels of glucose are excessively high, as occurs in diabetes mellitus. Extreme excitement or short-term stress can also elevate blood glucose, especially in the cat.
Related to diabetes mellitus and glucose metabolism is the ketone test square. The presence of ketones in the urine is always abnormal, and represents an abnormality is glucose metabolism. Ketonuria is a condition most commonly seen in poorly regulated diabetics.
Urobilinogen and Bilirubin are tests to help differentiate liver disease from destruction of red blood cells in the bloodstream.
A refractometer is used to determine urine specific gravity (SG). Here’s an easy way to understand specific gravity: Fill a drinking glass with water, and add a heaping tablespoon of mud. At this point, the specific gravity of the contents of the glass (muddy water) is pretty low, meaning you have a little mud in a lot of water.
Now, take that glass outside and leave it in the hot sun for a couple of days. As the water evaporates, and the mud stays behind, the contents of the glass gets thicker and thicker. That means that the specific gravity is going up. And, so it is with urine specific gravity.
When you’re watching the Super Bowl with a room full of your friends, and you’re drinking glass after glass of iced tea, the body tells the kidneys to get rid of the excess fluid you’re taking in, while continuing to rid the body of the usual amount of waste products. Thus, the urine specific gravity is quite low (lots of water, not much mud).
Alternatively, if you’re in the desert with no canteen, your body still has to get rid of the waste products it is producing, but there is no excess of water to spare. So, the kidneys form urine that has all the waste in a very small amount of urine, a high specific gravity. When your pet’s doctor asks you to withhold water from your pet before testing, he’s simulating those desert conditions, forcing the kidneys to “do their best”. How they perform on the specific gravity test tells the doctor a lot about your pet’s kidney health.
Next comes the microscopic portion of the urinalysis. This part is harder, and more time-consuming, but both parts must be performed if the doctor is to have a complete picture of the patient’s urinary tract and metabolic health.
The microscopic portion of the urinalysis can be performed either or both of two ways: stained, or unstained. No, we’re not talking about spilling the urine on the veterinarian’s clothing, we’re talking about applying stain to a thin layer of wet urine on a microscope slide. Some like it plain, some like it stained. There are advantages both ways and often a single specimen will be evaluated both ways.
On this part of the test we’re looking for cells, mostly. Normal cells that line the bladder are called transitional epithelium. Like any other cell in the body, they can become cancerous, so we’re keeping a sharp eye out for cancer cells under the microscope.
Sometimes bacteria can be seen. Normal urine is sterile, meaning that there are no microorganisms present. Therefore, if bacteria are seen, the immediate meaning is that the urinary tract is infected. Often the presence of bacteria will stimulate the doctor to order a culture of the urine. The purpose of a urine culture is to grow, identify and further refine the nature of the infection.
Another common finding under the microscope is the presence of casts. Granular casts are formed as cells or proteins take on the shape of the architecture of the interior of the kidneys. Seeing a few is normal, seeing a lot of them can indicate a serious problem.
Crystals are important when seen under the microscope. Sometimes they are indicators of stones, or there may be small numbers of crystals in a patient who hasn’t yet formed a stone, but the crystals tell the clinician that the patient is predisposed to forming stones. This early indicator may allow the doctor to change the patient’s diet, thus preventing a bigger complicating factor.
Some crystals mean worse news than others. For example, seeing crystals of ethylene glycol, antifreeze, usually means that the patient is in a lot of trouble.
Parasites. When you think of parasites, you usually don’t think of the urinary tract. However, there are parasites that live there, and the microscopic portion of the urinalysis is one of the best and least invasive ways to identify their presence.
The single most important thing to understand about urinalysis, chemical and microscopic, is that it’s not just about the urine and the urinary tract. Tests performed on the urine tell us much information about the function of the body as a whole.
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