Background Patients who display disruptive behaviours in the clinical encounter (the so-called ‘difficult patients’) may negatively affect doctors’ diagnostic reasoning, thereby causing diagnostic errors. The present study aimed at investigating the mechanisms underlying the negative influence of difficult patients’ behaviours on doctors’ diagnostic performance.

Methods A randomised experiment with 74 internal medicine residents. Doctors diagnosed eight written clinical vignettes that were exactly the same except for the patients’ behaviours (either difficult or neutral). Each participant diagnosed half of the vignettes in a difficult patient version and the other half in a neutral version in a counterbalanced design. After diagnosing each vignette, participants were asked to recall the patient's clinical findings and behaviours. Main measurements were: diagnostic accuracy scores; time spent on diagnosis, and amount of information recalled from patients’ clinical findings and behaviours.

Results Mean diagnostic accuracy scores (range 0–1) were significantly lower for difficult than neutral patients’ vignettes (0.41 vs 0.51; p<0.01). Time spent on diagnosing was similar. Participants recalled fewer clinical findings (mean=29.82% vs mean=32.52%; p<0.001) and more behaviours (mean=25.51% vs mean=17.89%; p<0.001) from difficult than from neutral patients.

Conclusions Difficult patients’ behaviours induce doctors to make diagnostic errors, apparently because doctors spend part of their mental resources on dealing with the difficult patients’ behaviours, impeding adequate processing of clinical findings. Efforts should be made to increase doctors’ awareness of the potential negative influence of difficult patients’ behaviours on diagnostic decisions and their ability to counteract such influence.