Quantifying the accuracy of a diagnostic test or marker.

BACKGROUND In recent years, increasing focus has been directed to the methodology for evaluating (new) tests or biomarkers. A key step in the evaluation of a diagnostic test is the investigation into its accuracy. CONTENT We reviewed the literature on how to assess the accuracy of diagnostic tests. Accuracy refers to the amount of agreement between the results of the test under evaluation (index test) and the results of a reference standard or test. The generally recommended approach is to use a prospective cohort design in patients who are suspected of having the disease of interest, in which each individual undergoes the index and same reference standard tests. This approach presents several challenges, including the problems that can arise with the verification of the index test results by the preferred reference standard test, the choice of cutoff value in case of a continuous index test result, and the determination of how to translate accuracy results to recommendations for clinical use. This first in a series of 4 reports presents an overview of the designs of single-test accuracy studies and the concepts of specificity, sensitivity, posterior probabilities (i.e., predictive values) for the presence of target disease, ROC curves, and likelihood ratios, all illustrated with empirical data from a study on the diagnosis of suspected deep venous thrombosis. Limitations of the concept of the diagnostic accuracy for a single test are also highlighted. CONCLUSIONS The prospective cohort design in patients suspected of having the disease of interest is the optimal approach to estimate the accuracy of a diagnostic test. However, the accuracy of a diagnostic index test is not constant but varies across different clinical contexts, disease spectrums, and even patient subgroups.