However, little kittens may give a false negative test result for many months - even when they are infected. (2ndchance.info)
• For example, an employer would keep a record of a blind positive test for five years and a blind negative test for two years. (transportation.gov)
Unfortunately, even this extensive testing is not always definitive, and negative testing does not mean the absence of heartworms 100 percent of the time. (texvetpets.org)