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Figure 1-1.

A model for clinical reasoning.

Constructing a differential diagnosis, choosing diagnostic tests, and interpreting the results are key skills for all physicians. The diagnostic process, often called clinical reasoning, is complex, and errors in reasoning are thought to account for 17% of all adverse events. Diagnostic errors can occur due to faulty knowledge, faulty data gathering, and faulty information processing. While this chapter focuses on the reasoning process, remember that the data you acquire during the history taking and the physical exam, sometimes accompanied by preliminary laboratory tests, form the basis for your initial clinical impression. Even with flawless reasoning, your final diagnosis will be wrong if you do not start with accurate data. You must have well developed interviewing and physical examination skills.

Clinicians generally use dual reasoning processes to work through a case. System 1 reasoning, relatively rapid and intuitive, is based on pattern recognition and involves matching the patient’s presentation to an illness script, a prior example stored in memory. System 2 reasoning is a slower process in which the clinician uses an explicit analytic approach. System 1 thinking predominates when an experienced clinician encounters a straightforward case, with system 2 predominating when the case is more complicated or the clinician is less experienced. Most of the time there is an unconscious blending of the two systems. Clinicians should be aware of common biases in clinical reasoning (Table 1-1) and reflect upon their reasoning processes, looking for potential errors. This chapter breaks down the reasoning process into a series of steps that can help you work through large differential diagnoses, avoid biases, and retrospectively identify sources of error when your diagnosis is wrong.

Table 1-1.Common biases in clinical reasoning.

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