Use of Likelihood Ratios in Evidence-based Clinical Decision Making

During the clinical decision making practitioners are often faced with performing diagnostic tests to solve the presenting problems seen in the patients. The diagnostic utility of a test has traditionally been described by technical terms such as sensitivity, specificity, and positive (PPV) and negative predictive value (NPV). Although well known, clinicians are frequently unclear about the concept and application of these terms in everyday evidence-based clinical decision making. Sensitivity and specificity, which are intrinsic properties of diagnostic tests, summarizes the characteristics of the test over a population. The PPV and NPV are greatly dependent on the population prevalence of disease, and thus they do not transferable to different patients or clinical settings. Besides, considering the fact that clinicians more often interested in knowing the extent to which a test result could confirm or exclude of a condition under consideration (posttest probability), these measures do not provide answers on this question. The likelihood ratios (LR) using the information contained in sensitivity and specificity are becoming increasingly popular for reporting the usefulness of diagnostic tests because this term provide an indication of posttest probability as a function of the pretest probability. In this article, clinical applications of LR are illustrated with some practical examples. Discussion is also included of the inherent limitations regarding diagnostic test characteristics.