Key concepts in clinical epidemiology: Estimating pre-test probability.

Appropriate medical management largely hinges on correctly diagnosing the underlying disease. Often, clinicians are faced with a dizzying array of accurate, albeit expensive and/or invasive diagnostic tests. What tends to be overlooked is that the probability of a disease once the test results are in (post-test probability) is a function of both the probability of the disease before the test was done (pre-test probability) and the diagnostic accuracy of the test. Clinicians need to be cognizant of inherent limitations in estimating pre-test probability and be more adept at finding ways to overcome these limitations. An accurate estimate of pre-test probability is pivotal. It guides the decision whether or not to conduct further testing, the choice of diagnostic test to perform, the interpretation of the test result, and the subsequent management of the patient's disease.