Evidence-based Medicine Toolkit

Applying the evidence 73DiagnosisIn diagnostic tests, you need to derive an estimate of your patients’pre-test probability, that is the likelihood that they have the disorderprior to doing the test. The prevalence from the study populationmay act as a guide. Trial data may exist which it generatessensitivities, specificities and LRs for clinical symptoms and signs;see the Rational Clinical Examination series in the Journal of theAmerican Medical Association, 1992–2001. This can be combinedwith the likelihood ratio of the test result to generate a post-testprobability.The term prevalence is applied to populations; pre-testprobability is applied to individuals.To calculate a post-test probability, you first need to convert yourpre-test probability into pre-test odds (see Altman 1991 for moredetails):Pre-test odds =pre-test probability(1 – pre-test probability)You can now multiply by the test result’s likelihood ratio to yieldthe post-test odds:Post-test odds = pre-test odds × LRIn turn, these post-test odds can be converted back into a posttestprobability:Post-test probability =post-test odds(post-test odds + 1)However, in the interests of simplicity, we suggest you either usethe nomogram on page 41 or the diagnostic calculator at http://www.cebm.net.