Heartworm Testing - Is it Good or Bad?
Excerpts from the Candian Vet Journal, CaN Vet J, Volume 41 Dec 2000, Page 935:
The principals of evidence-based medicine were used to determine the prevalence of heartworm infection in various dog populations and the effectiveness of screening these populations.
The evidence indicates that a heartworm diagnostic test applied to an asymptotic dog on preventative medication contributes little information regarding the heartworm infection status of that dog. However testing of a dog characterized as being high risk will provide clinically useful information eligibility of articles for critical review was based on the use of a gold standard for detection of heartworm, namely necropsy and the use of blinding the study design where the person performing the heartworm test was unaware of the necropsy results.
The specificity of most of the antigen tests appears to be very high, although the cited specificity's of 100% are likely a reflection of the low numbers of dogs tested. False-postive results can occur when adults have died less than 3mo prior to testing, as antigen may take this long to dissipate, when the monoclonal antibody binds to antigens from other nematodes or to other agents in the sera (especially if hemolysis or lipolysis is evident), and when nonspecific binding to a solid surface occurs.
Unreliability appears to be a problem with heartworm antigen test as many pratictioners are concerned with discrepant results on retesting of dogs. Data on reliability are sparse but appear to indicate that there may be some cause for concern. In a study conducted by Hoover et al. discordant replicated test results for 8 different tests were reported to have eliminated samples from further analysis.
A dog that has tested negative in the past and has been on heartworm preventive medication consistently during the parasite transmission season, whether in Canada or abroad, will have a pretest probability of being infected of virtually 0%.
Thus for dogs on a consistent preventative medication program, only 1 out of every 28 positive antigen test is likely to be a true positive.
Heartworm infection does not invariably lead to disease and death. In fact it is generally accepted, that low heartworm burdens < 20 worms are rarely of clinical significance (41
"It would appear from surveys that, in Canada, more dogs are euthanized because they tested positive than would ever become symptomatic or die from heartworm infection."
In order for practitioners to rationally interpret the results of a heartworm test, information on sensitivity and specificity of the test and the estimated pretest probability of infection in the dog have to be combined to obtain positive and negative predictive values.
The predictive values aid the practitioner in making a decision about the infection status of a dog. The positive predictive value (PPV) is the probabiltiy that a test-positive animal is truly infected with d. immitis.
If the pretest probability of heartworm infection is less than 5.0% the PPV drops off rapidly,making the decision to treat uncertain.
It is only when the pretest probability of heartworm infection is at least 3% to 5% that a positive test result, by itself,should change the practitioners opinion of the infection status of an unporotected,asymptomatic,as a PPV of 90% will be achieved.