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Vet Talk

What do Test Results Really Mean?
June 30, 2017 (published)
Carla Burris, DVM

Part 2


MRSA by CDC / NIAID

MRSA. Photo by CDC
It's easy to be confused about blood tests veterinarians and physicians recommend. Which ones are necessary? How much can I trust the results? What should I do if the results are positive? Why is 'positive' bad news and 'negative' good?

Easy question first: The positive = bad, negative = good thing messes with my mind sometimes too. You're not alone. Think of a blood test as a question. "Does my dog have heartworm disease?" If the test's answer is 'yes,' that's a positive result; your dog has heartworm disease. If the test's answer is 'no,' that's a negative. That seems so backwards! Until you consider some tests ask questions like, "Am I pregnant?" If you're on your second round of IVF, you want a positive result, but if the condom broke, you're probably wanting a negative. Just remember, the positive/negative is just a yes/no. You're bringing the value judgement of what that answer means.

How do you know if a test is worth running? There are some bad reasons, such as idle curiosity or a fearmongering magazine article. The key questions veterinarian ask themselves when considering testing are, "Can this test give clear results?", "Will the results of the test change anything?”, and “Is this a test in a sick pet or a healthy pet, and if healthy, why am I running it?” And they should be able to explain these answers to you.

The first question (“Can this test give clear results”) partly depends on the test itself and partly on your individual circumstances. Many of us want to believe that a “yes means yes, and a no means no”. You’re either pregnant (positive test) or you’re not pregnant (negative test). But that would require a perfect test, and we know that no test is perfect. Terminology alert! Blood tests are often described by their sensitivity and specificity. A sensitive test is going to do a better job of finding truly diseased patients -- it's less likely to mistakenly label a diseased patient as healthy (fewer false negatives). A perfect test would be 100 percent sensitive (no false negative results). If a very sensitive test is negative, you can be more confident that the patient doesn’t have that disease.

On the other hand, a specific test is going to do a better job of only flagging diseased patients: it's less likely to mistakenly label a healthy patient as diseased (fewer false positives). A perfect test would be 100 percent specific (no false positive results). If a very specific test is positive, you can be pretty confident that the patient really has the disease. Not only do sensitivity and specificity matter, but individual circumstances also matter.

The individual circumstances are where the art of medicine comes in. Maybe you live in an area where heartworm disease is prevalent (a large proportion of the population is infected) so you can't rule it out even if your coughing, panting cat tests negative. Maybe your puppy tested positive for parvo, but she was vaccinated last week and the test is catching those healthy antibodies rather than an actual infection. This is where you need to consult with your veterinarian. Few veterinarians can quote you exact numbers on sensitivity and specificity for every test (we look those up online when we need to), but you can expect us to know which tests are more trustworthy than others and in what circumstances. Ask us!

Did I lose you when I started talking about trusting results or being confident about them? I wrote up some examples (see link to Part II above the photo) to illustrate those concepts. If you're the kind of person who says, "Prove it!", there's your proof with numbers for the mathematical types.

The next big question is what the results of a test will mean to you and your pet. Will it actually change anything you do? Knowing that your dog's lethargy and vomiting is caused by that ox bone he wolfed down that got stuck in his intestines, rather than pancreatitis, means that he’ll probably need to go to surgery rather than be treated medically.

If the results of the test won't change what you choose to do, then it's probably not worth running it at all.

On the human side of things, routine PSA screening for prostate cancer in older men is probably not useful. Most prostate cancers are slow growing and take a long time to actually cause problems. If an 80-year-old man has a slightly high PSA, the odds are much higher he's going to die of a heart attack or car accident or lightning strike before a prostate tumor would be big enough to make peeing difficult. All that positive test did is add to his stress level at best, or prompted him to opt for unnecessary surgery and radiation at worst, with no gain to his length or quality of life. Was that test worth running?

This takes us to our third question: why? If your cat seems perfectly healthy, is current on vaccinations, and never has contact with other cats, annual testing for feline leukemia is silly. She's fine now and has a low risk of getting the disease, so why bother looking for something that's probably not there and never will be? Now if you're the director of an animal shelter with 20 cats looking for homes, you want to test all newcomers for feline leukemia before putting them in with the others -- you don't want to risk infecting everyone. If your pet is actually sick and has symptoms that could be due to a disease, that's a good reason to test for it. If you read a scary article about Ebola outbreaks in humans a continent away, that's no reason to run out and test yourself and your dog.

That's not to say that testing for peace of mind is all bad. If you're adopting a new puppy and it has a bout of diarrhea the day before you're due to bring him home, the odds are that it's just a transient bit of GI upset and the puppy's perfectly normal. But if your neighbor has a new puppy too and you want them to play together, it may be worth the time and money to run a parvo test. If it's negative (which is most likely), let the play dates commence worry free. If it's positive, then we get to worry about is it just a false positive from vaccination or an indication the poopy gates of hell are going to open soon? You can run that test; just make sure you're prepared to interpret the results with the help of your veterinarian.


 
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