Communicating the significance of radiologic test results: the likelihood ratio

The likelihood ratio is a useful but simple concept that helps the radiologist communicate to the clinician the significance of radiologic findings. It is defined as the ratio of the probabilities of a particular test result among patients with and without disease. Readily derivable from sensitivity and specificity, the likelihood ratio represents the degree to which a test result is positive or negative and serves as a proportionality constant that relates the pre- and posttest odds of disease in a linear manner. The linearity of the relationship permits the impact of a test result to be easily understood and allows for the integration of additional test results. The concept of likelihood ratio is explained in straightforward algebraic terms, and its practical application to clinical radiology is demonstrated by using several specific medical examples.