It's rare for heartworm tests to return either false positive or false negative results.  False positive test results happen when an animal has tested positive but does not have adult heartworms in his or her body at the time of testing. Your veterinarian will likely retest your pet or perform a different test if a single test reveals a positive result. False negative test results occur when a dog or cat is infected with adult heartworms and the test fails to detect their presence. This could be due to early infestations, very low numbers of parasites and infestation with mostly male worms.