• Thomas Agoritsas, Delphine S Courvoisier, Christophe Combescure, Marie Deom, and Thomas V Perneger.
• Division of Clinical Epidemiology, University Hospitals of Geneva, Gabrielle Perret-Gentil 6, 1211, Geneva 14, Switzerland. thomas.agoritsas@hcuge.ch
• J Gen Intern Med. 2011 Apr 1; 26 (4): 373378373-8.

BackgroundThe probability of a disease following a diagnostic test depends on the sensitivity and specificity of the test, but also on the prevalence of the disease in the population of interest (or pre-test probability). How physicians use this information is not well known.ObjectiveTo assess whether physicians correctly estimate post-test probability according to various levels of prevalence and explore this skill across respondent groups.DesignRandomized trial.ParticipantsPopulation-based sample of 1,361 physicians of all clinical specialties.InterventionWe described a scenario of a highly accurate screening test (sensitivity 99% and specificity 99%) in which we randomly manipulated the prevalence of the disease (1%, 2%, 10%, 25%, 95%, or no information).Main MeasuresWe asked physicians to estimate the probability of disease following a positive test (categorized as <60%, 60-79%, 80-94%, 95-99.9%, and >99.9%). Each answer was correct for a different version of the scenario, and no answer was possible in the "no information" scenario. We estimated the proportion of physicians proficient in assessing post-test probability as the proportion of correct answers beyond the distribution of answers attributable to guessing.Key ResultsMost respondents in each of the six groups (67%-82%) selected a post-test probability of 95-99.9%, regardless of the prevalence of disease and even when no information on prevalence was provided. This answer was correct only for a prevalence of 25%. We estimated that 9.1% (95% CI 6.0-14.0) of respondents knew how to assess correctly the post-test probability. This proportion did not vary with clinical experience or practice setting.ConclusionsMost physicians do not take into account the prevalence of disease when interpreting a positive test result. This may cause unnecessary testing and diagnostic errors.

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