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What Does Normal Mean? Understanding Your Pets Lab Results
Everyday pets receive laboratory testing both to determine health and also to help diagnose disease. Most of the time owners never get to see a copy of their pets' lab results. When they do, they are often confused as to what all the numbers mean. While it is beyond the scope of this brief article to discuss every lab test commonly performed, it is my hope that you can at least develop an appreciation for understanding how to interpret the results of some of the most commonly performed blood tests.
First, it's imperative that you understand what "normal" means. Every lab develops its own set of normal values based upon its patient population. Additionally, patients can have what appears to be an abnormal lab result yet be totally healthy. Why? This is because normal values are determined using a bell shaped curve. Using this curve, approximately 90% of "normal" patients will fall under the bell of the curve. This means that 5% of normal patients will fall outside the low end of the normal range and 5% will fall outside the high end of the normal range.
As an example, let's look at the BUN test, commonly used to determine kidney disease in dogs and cats. Normal values for most labs is 10-30. This means that 90% of normal pets will have a BUN value anywhere from 10 to 30. However, 5% of normal pets will have a lab value less than 10 and 5% of normal pets will have a lab value greater than 30.
Let's suppose your pet has a BUN value of 40, which is outside the normal range on the high end. How do you determine if this is normal for your pet or a sign of potentially serious kidney disease? This is where medical training comes in and the doctor relies upon his experience. If I saw this value in the lab report, there are several things I would do to determine if this is normal for your pet (in which case it's nothing to worry about,) or abnormal for your pet (in which case I would be concerned about the possibility of kidney disease.)
First I would want to look at BUN values from prior tests if those are available. If your pet always had BUN values close to 40, it's probably safe to say this is another normal test. If your pet has never had this test done, I would repeat it in one to two weeks and compare that value to today's value. Also I would look at other tests of kidney function (creatinine, phosphorus, complete urinalysis, etc.) and see if those were abnormal. If those were normal, it's likely that your pet's BUN value of 40 is nothing to worry about (although I would monitor the test every few months, especially if you have an older pet as kidney disease is more common in older pets.)
Additionally, we have to look at the magnitude of deviation from the normal value. A BUN of 40 is more likely to be normal then a BUN of 100. This is due to the wide variation in the normal range for this particular test as well as other things such as diet (which has nothing to do with kidney function) that can influence the test.
Because "normal" pets can have "abnormal" tests, I encourage owners of these pets not to worry about anything until more testing is done to determine if there are any potential problems with the pet.
With this background, let's take a look at some commonly performed tests and what they mean.
BUN (blood urea nitrogen) is a common test of kidney function. Unfortunately, it is also affected by other factors such as intestinal bleeding and dietary protein levels. With kidney disease, the BUN begins to slowly elevate. Mild elevations on a scale of 10 to 20 units can be normal for your pet if other testing of kidney function is also normal. Therefore, an elevated BUN by itself may not be clinically meaningful. However when combined with other tests of kidney function, an elevated BUN can alert the doctor for a potential problem with the pet's kidneys.
Creatinine is a much more sensitive test of kidney dysfunction. Even small elevations of this enzyme can indicate significant kidney problems. Because it is not affected by dietary protein, any elevation in creatinine levels should alert you to the strong possibility of underlying kidney disease.
There are many different pieces of information we can get from a urinalysis. One of the most important deals with kidney function and it's called the urine specific gravity. If the kidneys are functioning properly, they can either dilute or concentrate water that is presented to them by the bloodstream in the production of urine. If kidney function deteriorates, they are less able to produce dilute or concentrated urine, resulting in an abnormal specific gravity. As is true with blood testing, one abnormal specific gravity may be meaningless. Repeated abnormal values, combined with abnormal blood kidney testing, potentially signals a problem with the pet's kidneys.
While much rarer in dogs and cats than in people, diabetes can still be a serious disease. Fortunately it is usually easy to diagnose diabetes based upon laboratory testing and clinical signs.
There are two tests which are commonly used to diagnose diabetes in pets. The first is the blood glucose test. While these can become elevate due to other conditions such as stress (especially in cats that visit the doctor's office, when glucose levels can double and still be normal for that cat,) in general elevated blood glucose levels point to the strong possibility of diabetes.
The other test which can be used to help with the diagnosis of diabetes is a urinalysis. Normally, glucose does not appear in a normal pet's urine. However, as the blood glucose level elevates, some of it spills into the pet's urine. In serious diabetic conditions (called ketoacidosis,) glucose levels are very high as are levels of a fatty acid called ketones. Combining the results from a blood profile and urinalysis can help the doctor diagnose diabetes. In the rare case in which the results are in a gray area, another blood test which measures fructosamine can be performed. Elevated fructosamine levels may indicate that the elevated blood glucose levels are really due to diabetes and not to stress experienced by the pet during the office visit.
Even though many doctors incorrectly misdiagnose adrenal disease as liver disease (see below,) thankfully true liver disease is rare in most pets (although a bit more common in cats.) The best test for diagnosing liver disease is the ALT (formerly called SGPT) test.
With any sort of trauma/infection/inflammation/cancer to the liver, cell damage occurs and ALT leaks from the cells, causing elevated ALT levels in the blood. Small degrees of elevation can be normal, especially in older dogs (and pets taking medications such as corticosteroids or phenobarbital.) In cats, even small elevations can indicate serious liver disease, so these pets should have further testing. As is often the case with most blood tests, elevated levels do NOT tell us the cause of the disease, only that something is traumatizing the liver (infection, inflammation, benign or cancerous lesions, medications, etc.)
Overproduction of adrenal gland hormones (particularly cortisol) is one of the most common problems seen in older dogs. An elevated ALP (also called SAP) value was commonly indicates excess cortisol in the pet's body, either from overactive adrenal glands or from prescription steroid medications. While ALP values can also be elevated in pets with liver disease (especially cats,) it is most commonly elevated in pets with adrenal disease, especially if other liver tests are normal. Unfortunately, many doctors misdiagnose pets as having liver disease based upon an elevated ALP level, when in fact they have adrenal disease. This is one of the most common diagnostic mistakes I see in clinical practice, and it often results in dogs receiving unnecessary biopsies of their livers only to find out they have adrenal disease.
While there are no conventional medications to help pets with elevated ALP values, there are many herbs and homeopathics that can be used in these pets as we attempt to slow down the development of Cushing's disease which can happen if the adrenal glands continue to produce too much cortisol hormone.
Regular laboratory testing is important for your pet, both to determine his/her normal value, as well as to allow early diagnosis and treatment of potentially life-threatening diseases. Keep in mind that even "abnormal" blood and urine tests can be totally "normal" for your pet. If there is any doubt, simply repeating the test in1-2 weeks or getting a second opinion can allay any concerns.
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