The use of “overall accuracy” to evaluate the validity of screening or diagnostic tests

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Abstract
OBJECTIVE: Evaluations of screening or diagnostic tests sometimes incorporate measures of overall accuracy, diagnostic accuracy, or test efficiency. These terms refer to a single summary measurement calculated from 2 × 2 contingency tables that is the overall probability that a patient will be correctly classified by a screening or diagnostic test. We assessed the value of overall accuracy in studies of test validity, a topic that has not received adequate emphasis in the clinical literature. DESIGN: Guided by previous reports, we summarize the issues concerning the use of overall accuracy. To document its use in contemporary studies, a search was performed for test evaluation studies published in the clinical literature from 2000 to 2002 in which overall accuracy derived from a 2×2 contingency table was reported. MEASUREMENTS AND MAIN RESULTS: Overall accuracy is the weighted average of a test’s sensitivity and specificity, where sensitivity is weighted by prevalence and specificity is weighted by the complement of prevalence. Overall accuracy becomes particularly problematic as a measure of validity as 1) the difference between sensitivity and specificity increases and/or 2) the prevalence deviates away from 50%. Both situations lead to an increasing deviation between overall accuracy and either sensitivity or specificity. A summary of results from published studies (N=25) illustrated that the prevalence-dependent nature of overall accuracy has potentially negative consequences that can lead to a distorted impression of the validity of a screening or diagnostic test. CONCLUSIONS: Despite the intuitive appeal of overall accuracy as a single measure of test validity, its dependence on prevalence renders it inferior to the careful and balanced consideration of sensitivity and specificity.

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