Clinical trials as diagnostic tests

Concepts used in evaluating the results of diagnostic tests have been applied to clinical trials by several authors and each has reached the same conclusion: positive trials are more often falsely positive than would intuitively be expected. This conclusion is, however, based on assumptions that require close examination. First of all, it depends upon equating the power of a clinical trial with the sensitivity of a diagnostic test. Although it is possible to define circumstances in which the two are equivalent, decisions made on the basis of the results of clinical trials usually employ a broader definition of "true positive" than, it is shown, is implied by equating sensitivity with power. Secondly, it is assumed that one can speak meaningfully of the baseline "prevalence" of positive trials. The practical application of this concept can be shown to be extremely difficult. Thus, approaching clinical trials as if they were a type of diagnostic test is superficially appealing. However, this may result in misleading conclusions.