Likelihood Ratio: A Powerful Tool for Incorporating the Results of a Diagnostic Test Into Clinical Decisionmaking,☆☆,

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Abstract

[Hayden SR, Brown MD: Likelihood ratio: A powerful tool for incorporating the results of a diagnostic test into clinical decisionmaking. Ann Emerg Med May 1999;33:575-580.]

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WHAT ARE LIKELIHOOD RATIOS?

The LR is a powerful measure of the accuracy of a diagnostic test. It is the ratio of the probability of a given test result in patients with disease to the probability of the same test result in patients without disease. The LR for a given test result indicates how much that result will raise or lower the probability of disease. It can be used with a nomogram developed by Fagan2 (Figure 2) to establish the posttest probability corresponding to any pretest probability and for any test result.

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HOW ARE LIKELIHOOD RATIOS DERIVED FROM PUBLISHED DATA?

LRs are not always directly reported in studies of the accuracy of diagnostic tests. It is, however, usually possible for the reader to calculate them using data commonly reported in such studies.

Authors report the results of studies in a variety of ways. When only 2 test results are possible, they are commonly labeled “positive” versus “negative.” In such cases, the results of studies on accuracy are frequently reported as “sensitivity,” “specificity,” “positive predictive value,” and

HOW ARE LIKELIHOOD RATIOS APPLIED IN CLINICAL DECISIONMAKING?

A diagnostic test that is useful would ideally have either a high LR or a low LR. As the LR approaches 1.0, the utility of the test decreases to zero. Once the LR is known for a given diagnostic test result, use of the nomogram developed by Fagan2 is the simplest method for the clinician to arrive at a posttest probability. Use of the nomogram avoids the necessity of an extra set of computations and streamlines the process of using LRs. In most cases, practitioners formulate their clinical

EXAMPLE

Consider the case of an 8-year-old boy who presents to the ED complaining only of sore throat and fever. On examination he has an erythematous pharynx with mild anterior cervical adenopathy. Because physicians may overestimate GABHS infection,5, 6 you have adopted a strategy that uses diagnostic testing as opposed to empiric treatment for pediatric patients with sore throat.7, 8 You are able to arrange good follow-up for your ED population, and your clinical estimate of the likelihood of a

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Address for reprints: Stephen R Hayden, MD, Department of Emergency Medicine, UCSD Medical Center, Mail Code 8676, 200 West Arbor Drive, San Diego, CA 92103.

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