Peer-reviewed veterinary case report
How blocked antigen affects heartworm tests in dogs and cats
By Little, Susan et al.·Published in Parasites & vectors·2018·Department of Veterinary Pathobiology, United States·View original on PubMed →
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Original publication title: Prime detection of Dirofilaria immitis: understanding the influence of blocked antigen on heartworm test performance.
- Species:
- dog
Plain-English summary
A dog with a suspected heartworm infection may test negative for the disease even if it is present. This can happen because the heartworm antigen (a substance indicating infection) might not be detectable in some cases. Researchers found that heating the blood samples can reveal the antigen, turning negative tests into positive ones. This method was shown to be effective in dogs that had previously tested negative but were actually infected. However, routine heating of all samples isn't recommended, especially for dogs that consistently receive heartworm prevention, as they are unlikely to be infected.
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Abstract
Detection of circulating antigen of Dirofilaria immitis has been a mainstay of identifying heartworm infection in clinical practice for the past three decades. Several validated commercial antigen tests have very good sensitivity, specificity, and positive predictive values, especially when used in patients for which heartworm infection is likely. In some dogs and cats infected with heartworm, antigen may not be available for detection although present in the patient sample; heat pretreatment of these samples reveals the antigen, changing the false negative to positive. This phenomenon was documented in the literature in the 1980s but subsequently overlooked by the heartworm research community for many years. In this review, we provide a summary of the current understanding of the role of heat reversal in diagnosing heartworm infection. This additional diagnostic step is most important for patients in which heartworm infection is likely, such as dogs or cats in an endemic area with an inconsistent history of heartworm preventive use, or dogs with a prior diagnosis of heartworm infection that were recently treated. To illustrate the concept, we share a summary of results from canine samples tested at the state veterinary diagnostic laboratory in Oklahoma, USA in 2017 by modified Knott test and by commercial antigen test before and after heat treatment of samples; in this sample set, heat treatment changed all D. immitis microfilaria-positive but antigen-negative samples to antigen-positive. Pet dogs with a history of consistent preventive use are unlikely to become positive with heat pretreatment; for that reason, routine pretreatment of all samples tested in a veterinary practice is not recommended. We also review known causes of false negative and false positive results on heartworm antigen tests that, although uncommon, can complicate accurate diagnosis in individual patients. Together, this review provides a primer to aid understanding of strategies that can enhance accurate diagnosis of heartworm infection in veterinary practice and clinical research.
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Search related cases →Original publication on PubMed: https://pubmed.ncbi.nlm.nih.gov/29554955/