Characteristics of good diagnostic studies.

Whether or not patients are better off from undergoing a diagnostic test will depend on how test information is used to guide subsequent decisions on starting, stopping, or modifying treatment. Consequently, the practical value of a diagnostic test can only be assessed by taking into account subsequent health outcomes. In the appraisal of diagnostic test studies, it is essential to discriminate between studies that report on the accuracy of a diagnostic test and studies that report on health outcomes of strategies that incorporate diagnostic tests. In a study that reports on diagnostic accuracy, a cohort of patients is subjected to at least two diagnostic tests: the index test and the reference test, the latter usually being the best method available to detect the target condition. The accuracy of the index test can be expressed in terms of sensitivity, specificity, or likelihood ratios. Studies that compare two or more strategies that incorporate diagnostic tests as well as therapeutic interventions should be approached differently. Such studies do not require expression of test accuracy in terms of sensitivity and specificity. The merit of diagnostic tests evaluated in such studies can be expressed by comparing relevant outcomes of both strategies. The effectiveness of such strategies can be compared similarly as the effectiveness of treatment. However, due to the fact that the effect of a diagnostic test on health outcome is not as direct as the effect of treatment on health outcome, the design of outcome studies reporting on diagnostic tests requires special attention. It is important to establish a clear link between the result of the test under study and subsequent therapeutic management. Furthermore, trial efficiency can be improved by moving the point of randomization from the decision point, whether or not to test, to the point where a decision has to be made regarding what to do with the positive test results.