Assessment of the accuracy of diagnostic tests: the cross-sectional study.

In diagnostic accuracy studies, the contrast of interest can be one of the following: one single test contrast; comparing two or more single tests; further testing in addition to previous diagnostics; and comparing alternative diagnostic strategies. The clinical diagnostic problem under study must be specified. Studies of "extreme contrasts" (as early phase evaluations) and studies in "clinical practice" settings (assessing clinical value) should be distinguished. Design options are (1) survey of the total study population, (2) case-referent approach, or (3) test-based enrollment. Data collection should generally be prospective, but ambispective and retrospective approaches are sometimes appropriate. In addition to determinants of primary interest [the test(s) under study] possible modifiers of test accuracy and confounding variables must be specified. The reference standard procedure should be independent from the test results. Applying a reference standard can be difficult in case of classification errors, lack of a clear pathophysiologic concept, incorporation bias, or invasive or complex investigations. Possible solutions are: an independent expert panel, and the delayed type cross-sectional study (clinical follow-up). Also, a prognostic criterion can be chosen. For studies to be relevant for practice, inclusion criteria must be based on "intention to diagnose" or "intention to screen." The recruitment procedure is preferably a consecutive series of presenting patients or a target population screening, respectively. Sample size estimation should be routine. Analysis has to be focused on the contrast of interest. Estimating test accuracy and prediction of outcome need different approaches. External (clinical) validation requires repeated studies in other, similar populations. Also, systematic reviews and meta-analysis have a role. To enable readers of diagnostic research reports to evaluate whether methodological key issues were addressed, authors are advised to follow the STARD guidelines.