Canine Heartworm Testing

The following are possible interpretations of positive and negative test results. You should consult with your private veterinarian for specific advice concerning your pet.

Heartworm Negative

A negative result almost always means that your dog does not have an active infection with adult heartworms at this time. It is probably safe for your private veterinarian to start your dog on preventative medication. Heartworm is spread by bites from infected mosquitoes and, because of the time lapse between being bitten by an infected mosquito an the appearance of enough heartworm proteins in your dog’s blood to make the test register positive, there is a small possibility of a false negative test result. Consult your private veterinarian regarding the appropriate preventative medication for your dog and the interpretation of the test results.

Heartworm Positive

This means that your dog probably has adult heartworms in his/her blood today. Do not give your dog any heartworm preventive medication if today’s test was positive. False positive tests occasionally occur, so it is important that you consult your veterinarian for follow-up advice. A second test will probably be done. Heartworm is a serious, even life threatening disease, but it can be treated. If it is caught before your dog’s heart and lungs are damaged by the parasite, most dogs do extremely well. The cost of treatment varies greatly depending upon the amount of diagnostic work needed and the damage the parasite has caused before being detected. Consult your private veterinarian.

What is a false negative test?

This happens when your dog is infected with adult heartworms and the test fails to detect their presence. Early infestations, very low numbers of parasites, and infestation with mostly male worms are the most common causes of false negatives. False negative heartworm tests are rare.

What is a false positive test?

A false positive test result occurs when the test comes out positive, but your dog does not have adult heartworms in his/her body at this time. It is impossible to tell the difference between a false positive and a true positive result with a single test, so your veterinarian will probably retest the dog or do a different test. False positives are unusual.