Does prevalence matter to physicians in estimating post-test probability of disease? A randomized trial.
BACKGROUND
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.


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


METHODS
Randomized trial.


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


METHODS
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).


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Division of Clinical Epidemiology, University Hospitals of Geneva, Gabrielle Perret-Gentil 6, 1211, Geneva 14, Switzerland. thomas.agoritsas@hcuge.ch
Thomas
T
Agoritsas
Delphine S
DS
Courvoisier
Christophe
C
Combescure
Marie
M
Deom
Thomas V
TV
Perneger
D000328:Adult; D003951:Diagnostic Errors / Q000517:prevention & control; D003955:Diagnostic Tests, Routine / Q000379:methods* / Q000592:standards / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D010820:Physicians / Q000592:standards; D015995:Prevalence; D011336:Probability; D011795:Surveys and Questionnaires