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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 will focus on the reasoning process, remember that the data you acquire through your history and 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 often use a combination of 2 reasoning processes: non-analytical/intuitive and analytical. The intuitive process is rapid and consists of an unconscious match to examples stored in memory, while the analytical process is slow, logical, and rule-based. 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.


Figure 1-1.

A model for clinical reasoning.

Step 1: Identify the Problem

Be certain you understand what the patient is telling you. Sometimes “I’m tired” means “I become short of breath when I walk” and at other times means “My muscles are weak.” Construct a complete problem list consisting of the chief complaint, other acute symptoms and physical exam abnormalities, chronic active problems (such as diabetes or hypertension), and important past problems (such as history of bowel obstruction or cancer). Problems that are likely to be related, such as shortness of breath and chest pain, should be grouped together. It is necessary to accurately identify the problem every time you evaluate a patient.

Step 2: Frame the Differential Diagnosis

The differential diagnosis should be framed in a way that facilitates recall. It might be possible to memorize long lists of causes, or differential diagnoses, for various problems. However, doing so ...

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