Using evidence-based medicine in clinical practice.

This article discusses the cognitive tendencies that lead to inaccurate appraisal of the value of tests or treatments, the role of evidence-based medicine in correcting them, and the need for the primary physician to provide an evidence-based perspective. True clinical outcomes and levels of evidence are defined. The clinical epidemiology of testing is reviewed, with emphasis on sensitivity, specificity, and positive and negative predictive values. These are followed by criteria for usefulness in screening. Treatment evaluation is discussed in terms of absolute risk reduction and number needed to treat.