Background Flaws in clinical reasoning are present in most diagnostic errors and occur even when physicians have enough knowledge to solve the problem. Deliberate reflection has been shown to improve diagnoses. The sources of faulty reasoning and how reflection counteracts them remain largely unknown.

Objective To explore the causes of faulty reasoning and the mechanisms through which reflection neutralises them by investigating the influence of salient distracting clinical features on diagnostic decision-making.

Design and setting In a prior study, 34 internal medicine residents and 50 medical students of the Erasmus Medical Centre, Rotterdam, diagnosed four clinical cases by means of non-analytical reasoning and four by reflective reasoning. In the secondary analysis of the data presented here, five internists independently evaluated the diagnoses and examined the nature of the diagnostic errors in relation to case features that gave rise to these errors.

Main outcomes Frequency of incorrect diagnoses caused by salient distracting features made through reflective and non-analytical reasoning.

Results Among residents, reflective reasoning (Mean diagnostic accuracy score (M)=2.09, 95% CI 1.77 to 2.40) led to a significantly higher number of correct diagnoses than non-analytical reasoning (M=1.71, 95% CI 1.37 to 2.04; p=0.03). This higher diagnostic accuracy was associated with fewer incorrect diagnoses triggered by salient distracting clinical features (M=0.47, 95% CI 0.26 to 0.68) compared with non-analytical reasoning (M=0.85, 95% CI 0.59 to 1.11; p=0.02). Students did not benefit from reflection to improve diagnoses.

Conclusion Salient features in a case tend to attract physicians' attention and may misdirect diagnostic reasoning when they turn out to be unrelated to the problem, causing errors. Reflection helps by enabling physicians to overcome the influence of distracting features. The lack of effect for students suggests that this is only possible when there is enough knowledge to recognise which features discriminate between alternative diagnoses.