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Table 5 Patientsa most likely to benefit from heat pretreatment of samples prior to antigen testing for Dirofilaria immitis

From: Prime detection of Dirofilaria immitis: understanding the influence of blocked antigen on heartworm test performance

Patient type Reason
Dogs in endemic areas not on preventive, especially young dogs Allows detection of infection earlier than testing non-pretreated samples.
Dogs with an inconsistent history of preventive use Macrocyclic lactones given intermittently may begin to kill adult worms, leading to inflammation and immune complex formation. Microfilariae are also less likely to be detected in patients that have received preventives.
Heartworm-infected dogs recently treated with adulticide or managed only with preventive and doxycycline Inflammation induced by dead and dying worms may lead to immune complex formation that masks antigen, preventing detection in unheated samples. Residual antigen may be detected both with and without heat pretreatment of samples in dogs recently treated for heartworm infection.
Dogs with microfilariae detected but that are antigen-negative If microfilariae are D. immitis, then heat pretreatment is likely to change the result to true positive.
Cats in endemic areas not on preventive Blocked antigen is very common in infected cats, particularly early in infection when immune response is most pronounced.
  1. aExercise caution in interpreting heartworm antigen test results, with or without heat pretreatment, on samples from dogs living in areas where infection with Angiostrongylus vasorum or Spirocerca lupi is common; both have been shown to cause false positive results on some antigen tests (see Table 8)