Heartworm tests use a blood sample to detect either a specific antigen released by mature female heartworms or the offspring (microfilariae) of these worms.

It generally takes 6 months from the time the pet is infected with heartworm larvae by a mosquito bite until female worms can be detected. The administration of heartworm preventives to dogs with mature female worms can delay their detection for up to 9 months after infection.

This means that a dog may test “negative” for heartworms, even though it has adult worms in the heart and blood vessels of the lungs.