Evaluation of heartworm immunodiagnostic tests

Charles H. Courtney From the Department of Infectious Diseases, College of Veterinary Medicine, (Courtney) and the Department of Statistics (Cornell), University of Florida, Gainesville, FL 32610.

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John A. Cornell From the Department of Infectious Diseases, College of Veterinary Medicine, (Courtney) and the Department of Statistics (Cornell), University of Florida, Gainesville, FL 32610.

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Summary

In this report, the use of appropriate statistical methods for the evaluation of heartworm immunodiagnostic tests is discussed. The evaluation of these tests is complicated by factors causing variation in sensitivity, specificity, accuracy, and predictive values of positive and negative test results. The primary sources of inconsistency are variation in the prevalence of heartworm infection among populations of dogs and the sensitivity of immunodiagnostic tests to various categories of heartworm infections (ie, patent, immune-mediated occult, unisex occult, and immature occult). Sample size (ie, number of dogs tested) affects the confidence limit values of sensitivity and specificity. At least 100 dogs should be used in each testing group (infected and uninfected) to generate values of sensitivity or specificity within reasonably narrow confidence limits. Use of more than 200 dogs in each testing group contributes little to further narrowing of confidence limits. The selection of appropriate statistical tests for comparison of tests or comparison of the sensitivity or specificity of a single diagnostic test to various categories of heartworm infections is critical. The McNemar paired χ2 test is appropriate for comparison of diagnostic tests, but it must be done by use of duplicate sera from each animal. A χ2 test of independence, or, in the case of a small sample size, the Fisher exact test, is appropriate for comparing the sensitivity or specificity of a single diagnostic test to various categories of heartworm infection.

Summary

In this report, the use of appropriate statistical methods for the evaluation of heartworm immunodiagnostic tests is discussed. The evaluation of these tests is complicated by factors causing variation in sensitivity, specificity, accuracy, and predictive values of positive and negative test results. The primary sources of inconsistency are variation in the prevalence of heartworm infection among populations of dogs and the sensitivity of immunodiagnostic tests to various categories of heartworm infections (ie, patent, immune-mediated occult, unisex occult, and immature occult). Sample size (ie, number of dogs tested) affects the confidence limit values of sensitivity and specificity. At least 100 dogs should be used in each testing group (infected and uninfected) to generate values of sensitivity or specificity within reasonably narrow confidence limits. Use of more than 200 dogs in each testing group contributes little to further narrowing of confidence limits. The selection of appropriate statistical tests for comparison of tests or comparison of the sensitivity or specificity of a single diagnostic test to various categories of heartworm infections is critical. The McNemar paired χ2 test is appropriate for comparison of diagnostic tests, but it must be done by use of duplicate sera from each animal. A χ2 test of independence, or, in the case of a small sample size, the Fisher exact test, is appropriate for comparing the sensitivity or specificity of a single diagnostic test to various categories of heartworm infection.

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