False negative antigen tests in dogs infected with heartworm and placed on macrocyclic lactone preventives.

Background: Dogs with chronic inflammation, including those with heartworm being managed with macrocyclic lactones and doxycycline (slow kill, SK), may develop immune complexes that block detection of Dirofilaria immitis antigen on commercial tests.

Methods: To determine if SK could result in development of false-negative antigen tests, we collected serum samples from dogs that had been diagnosed with heartworm by antigen detection, with or without confirmation by detection of D. immitis microfilariae, placed on monthly macrocyclic lactones and doxycycline, and that later tested negative on an antigen test, and then tested them for antigen of D.