To our surprise, our patient's test result generates this indeterminate likelihood ratio of only 1. The test we'd have judged very useful, based on the old sensitivity-specificity approach, really hasn't been helpful in moving you toward the diagnosis. At this and every other stage in the diagnostic process, we must decide whether to test further, treat, or abandon a diagnosis. These limits are called thresholds. Between these two thresholds, we continue to carry out diagnostic tests in the hope that their results will send us across one of the thresholds, which are functions of such factors as the precision and accuracy of the diagnostic test and the risks and benefits of both the test and the treatment.
Thresholds vary among diseases and individual patients.
This nomogram, when coupled with test-treatment thresholds, also provides clinicians with an additional approach to determining appropriate pretest probabilities. This time, the straight edge is anchored on the likelihood ratio for the diagnostic test result and rotated until it intersects the post-test probability that corresponds to the appropriate threshold. The key definitions in this EBM Note have been added to the Glossary, and future notes will present other elements of the science of the art of clinical diagnosis in an effort to render it more effective and efficient and to promote improvements in the care of patients. David L. Sackett, MD Sharon Straus, MD University of Oxford Oxford, England, UK References 1. Sackett DL, Richardson WS, Rosenberg W, Haynes RB, eds.