False Positive, False Negative

Run a test. Get a result. Sounds simple enough, doesn’t it.

However, the wicket gets sticky when you realize that every test we run requires interpretation. Few are simply “yes” or “no,” “black” or “white.”

When results are dramatically different from what we expect, suspicions rise of “false positive” or “false negative.”

A false positive result sometimes occurs when something in the patient’s body interferes with the normal function of the test. When the test result is suspect, another test must be performed to confirm or deny the first test’s result.


This is a picture we took of Freckles when he was brand new (but don't ask me which one is him!)

Freckles, the inspiration for this post, is an excellent example. Approaching 2 years of age Freckles came in for a routine examination, vaccinations, semiannual heartworm test and stool test for intestinal parasites. Everything went smoothly until we read his heartworm test. Despite getting heartworm preventive right on schedule every month, Freckles’ test was positive, indicating he was infested with heartworms!

As we have written previously, all heartworm preventives have a certain failure rate.  No medication is perfect. The more common scenario is that a patient misses one or more doses or fails to be dosed on schedule every 30-31 days. So, it wasn’t totally out of the question that Freckles could have heartworms, just unlikely.


An example of a positive heartworm test result.

We checked his purchase history, and his Mom had bought more than enough Revolution to account for each month’s dosing. Of course, buying heartworm preventive and administering heartworm preventive aren’t the same thing, but we knew this Mom to be quite conscientious, plus, she assured us that his doses had not ever been late.

We obtained another blood sample and sent it to a reference laboratory for a confirmation test. If that test was also positive, we would be convinced that, indeed, Freckles would require a heartworm treatment to remove Dirofilaria immitis from his body.

If the confirmation test was negative, we would be strongly suspicious that the first test was in error.

Fortunately, the second test was negative, leading us to conclude that Freckles was triggering a positive result with our in-house heartworm test because of something in his body that most dogs don’t have. Which was odd, because he had a heartworm test six months prior that was negative.

It was also odd because canine antigen heartworm testing has a high specificity. Specificity is a measure of the likelihood of a positive result actually being true. Heartworm tests measure molecules from the uterus of the female adult heartworm. That’s pretty specific! So, a test with high specificity means that a positive result is highly believable.

Another characteristic of the quality of a test is sensitivity. Heartworm tests typically give a positive result with as few as 1-3 adult heartworms. That is approximately the number of heartworms required to release enough molecules of the female heartworm’s uterus escaping her body and entering circulating blood. Therefore, this test has a high sensitivity and would be expected to give a positive result on every dog with 3 or more sexually mature adult heartworms, and a negative test result is highly believable.

Put another way, a test with a high specificity means you can almost always believe in a positive result and a test with a high sensitivity means you can almost always believe in a negative result. Mnemonics used to stimulate memory of these principles are SPPIN and SNNOUT. A SPecific test, when Positive, rules IN a disease. A SeNsitive test, when Negative, rules OUT a disease.

Here are some examples of false negative tests that occur commonly in practice.

Whipworms are intestinal parasites of dogs which notoriously produce small numbers of eggs and produce them sporadically. A fecal flotation test is designed to detect eggs of intestinal parasites. That test, performed at a time when no eggs were being produced, in a dog known to harbor whipworms, would give us a false negative result. Test enough times, however, and one would expect to eventually confirm the worms’ presence.

Researchers know that as many as one-third of cats harboring Feline Leukemia Virus (FeLV) sequester it in their bone marrow, where a test on a sample of peripheral (arterial or venous) blood would not detect it. So, an in-house blood test for the disease would deliver a false negative, and a positive bone marrow test would confirm our suspicion that the kitty was FeLV-positive.

False positive and false negative test results can be complicated to understand, but it is important to comprehending your pet’s doctor’s interpretation when results seem confounding.

See you next week, Dr. Randolph.


  1. Margaret Walbroel says:

    Hi doctor! My dog is on heart guard and just received a positive heartworm test. The first vet said he had trouble getting a blood sample but got enough. The test he said was “cloudy” so he sent it off for lab confirmation. The lab confirmed. I took him to a second vet who tells me his test is negative. Now I’m confused! Should I request to know which test was used? Would you be more inclined to believe the first vet as he received confirmation with a lab?

    • First, have one of the doctors perform a microfilaria test 30 days after a dose of Heartgard. If your dog has microfilariae, odds are good (it is NOT 100% slam dunk) he has adult heartworms, and you can proceed to further diagnostics or to heartworm treatment. What you need next, if the microfilaria test is negative and if you haven’t gotten it already, is a “DIROCHEK” brand heartworm test. The manufacturer of our heartworm tests, Witness, sold and marketed through Zoetis Animal Health, will perform a Dirochek confirmation test at no charge. Your pet’s doctor’s test brand may do the same thing. If not, go ahead and pay for it. A Dirochek test has very little false positives or negatives. Both tests require drawing an additional blood sample. Thanks for reading, Dr. Randolph.

  2. B says:

    Hi There!

    Today at the vet my dog tested negative for heartworms but there was a faint line for positive? This has happened in the past when we first adopted him but came back fully negative when sending it off for further testing.


    • Help? Sorry, it’s not that simple. Heartworm testing can be incredibly complex. I’m going to make some assumptions, though. If the confirmation test your veterinarian sent off was a well test, Dirocheck is considered the gold standard brand, and you got a negative, you have little or nothing to worry about. Watchful waiting is usually employed. You could order a chest radiograph, but that’s probably overkill. Your veterinarian’s brand of heartworm test might pay for another confirmation test. Many brands do. Frustrating, isn’t it? It’s frustrating for us, too. Write back and let us know how it goes.

  3. Microfilaria says:


    During his yearly exams my boy who is 6 years old showed a “weak positive” for heartworm, He’s been on heartgard since the beginning and has not missed a dose. Did an Abaxis test and the result came back negative. Not even a faint line- nothing. Will be sending blood work away for a third test but what does your vet gut tell you about this scenario? and should he be given something in the meantime besides his regular heartgard dose just in case. thank you.

    • Read this article, please. In addition to the third blood test, your dog could be referred for echocardiogram (cardiac ultrasound), which can sometimes pick up the presence of heartworms, or even a chest X-ray, which your local veterinarian can read or have sent out to a board-certified radiologist. The latter is unlikely to be helpful, however, because small heartworm burdens may not elicit enough response from the body to be seen on the radiograph. Dirocheck is widely (but not universally) considered the gold standard of heartworm tests. The veterinarians at Heartgard’s manufacturer can also guide your veterinarian. It sounds like your veterinarian is right on top of things! Thanks for reading.

  4. Susan L. Carley says:

    “Heart Guard” does not work for prevention for heart worm. Our dog had heart worm is a rescue when we found out we immediately started treatment which is to give preventative so we gave her heart guard then just before her first needle she still had the babies in the blood stream (which is dangerous for the dog) it should of killed the Microfilaria and didn’t so we had to change to a different medication. Advantage and checked two months later and the Microfilaria was gone. Many people are not aware that the preventive is not working because the worm has become immune to it. People are being blamed for being late with the preventative etc .

    • I’m afraid there are some flaws in your understanding of the life cycle of Dirofilaria immitis, commonly referred to as heartworm. “Preventive” is not “treatment.” Click here to read an article that explains the difference, as what you’re referring to is “slow kill,” which is discouraged by the American Heartworm Society. The dose of active ingredient in Heartgard is too low to kill microfilaria in some patients, so it’s not unusual for microfilaria to continue to be seen. Also, since Heartgard is not killing the adult heartworms, microfilaria are continuing to be produced, yet another problem with slow kill. Advantage is a flea and tick killing medicine, and has no effect on adult heartworms or their microfilaria. You may be confusing Advantage with Advantage Multi, a totally different product, but which is also a heartworm preventive. You are correct in stating that there is a problem of resistance to heartworm preventives, and many, many experts believe that some of that resistance is being caused by the use of heartworm preventives for “slow kill.”

  5. Melissa says:

    My dog just got diagnosed with heart worm but has been on preventive medicine. Our other two dogs were negative. She also had tick fever at the time the test was taken. She completed the medicine for the tick fever and has just received her first shot for heartworm. She is not coughing or trying to clear her throat like they said she would do. Is it possible the tick fever interrupted the heartworm test in some way?


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