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Does Prevalence Matter to Physicians in Estimating Post-test Probability of Disease? A Randomized Trial

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The 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.


To assess whether physicians correctly estimate post-test probability according to various levels of prevalence and explore this skill across respondent groups.


Randomized trial.


Population-based sample of 1,361 physicians of all clinical specialties.


We 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).


We 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.


Most 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.


Most 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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The authors thank Véronique Kolly, Research Nurse at the University Hospitals of Geneva, for her help and guidance in conducting the survey.


The survey was funded by The Research and Development Program of University Hospitals of Geneva. There was no external funding.


Part of this paper was presented as an abstract and poster at the 78th Annual Meeting of the Swiss Society of Internal Medicine, Basel, Switzerland, in May 2010

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Correspondence to Thomas Agoritsas MD.

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Agoritsas, T., Courvoisier, D.S., Combescure, C. et al. Does Prevalence Matter to Physicians in Estimating Post-test Probability of Disease? A Randomized Trial. J GEN INTERN MED 26, 373–378 (2011).

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