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Most diagnostic errors involve faulty diagnostic reasoning. Consequently, the authors assessed the effect of querying initial hypotheses on diagnostic performance.In 2007, the authors randomly assigned 67 first-year medical students from the University of Calgary to two groups and asked them to diagnose eight common problems. The authors presented the same primary data to both groups and asked students for their initial diagnosis. Then, after presenting secondary data that were either discordant or concordant with the primary data, they asked students for a final diagnosis. The authors noted changes in students' diagnoses and the accuracy of initial and final diagnoses for discordant and concordant cases.For concordant cases, students retained 84.2% of their initial diagnoses and were equally likely to move toward a correct as incorrect final diagnosis (6.9% versus 8.9%, P = .3); no difference existed in the accuracy of initial and final diagnoses: 85.9% versus 84.0% (P = .4). By contrast, for discordant cases, students retained only 23.3% of initial diagnoses, change was almost invariably from incorrect to correct (76.3% versus 0.4%, P < .001), and final diagnoses were more accurate than initial diagnoses: 80.7% versus 4.8% (P < .001). Overall, no difference existed in the accuracy of final diagnoses for concordant and discordant cases (P = .18).These data suggest that querying an initial diagnostic hypothesis does not harm a correct diagnosis but instead allows students to rectify an incorrect diagnosis. Whether querying initial diagnoses reduces diagnostic error in clinical practice remains unknown.