Circulating heartworm antigen appears in the blood as early as five months post-infection in a small percentage of dogs, but most dogs are not antigen positive until seven months post-infection. Yes, contrary to popular belief, a dog infected six months previously can be negative on an antigen test.
Can heartworm tests be wrong?
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.
How common is a false negative heartworm test?
Blocked Antigen Causes False-Negative Heartworm Test Results The antigen test is generally considered the more sensitive test, but studies estimate that 6.0% to 38.7% of microfilaria-positive samples produce negative antigen test results.
How accurate are heartworm tests for dogs?
Depending on prevalence, the positive predictive value of the heartworm antigen test ranged from 15 to 54% and negative predictive value from 99 to 99.9%. For a hospital testing 1,000 dogs per year, false-positive test results may vary from 24 to 27 dogs.
How long can a dog have heartworms before testing positive?
The earliest that microfilariae can be detected in a dog's bloodstream is about 6 months after it is bitten by an infected mosquito (because it takes about that long for the heartworms to develop from infective larvae into adults that mate and produce microfilariae).
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