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Constructing a differential diagnosis, choosing diagnostic tests, and interpreting the results are key skills for all physicians and are some of the primary new skills medical students begin to learn during their third year. The diagnostic process, often called clinical reasoning, is complex, but it can be broken down into a series of steps, diagrammed in Figure 1–1.

Step 1: Data Acquisition

Data you acquire through your history and physical exam, sometimes accompanied by preliminary laboratory tests, form the basis for your initial diagnostic reasoning. Your reasoning will be faulty unless you start with accurate data, so the prerequisite for obtaining valid data is well developed interviewing and physical examination skills.

Step 2: Accurate Problem Representation

This step consists of developing a “problem synthesis statement,” a concise, single sentence summary of the main clinical problem and its associated context.

Clinical problems are symptoms, physical findings, test abnormalities, or health conditions for which diagnostic evaluation could be undertaken. The problem synthesis statement is meant to focus on the patient's most important problem, usually the chief complaint.

Context refers to pivotal points, generally one of a pair of opposing descriptors used to compare and contrast diagnoses or clinical characteristics; for example, old versus new headache, unilateral versus bilateral edema, smoker versus nonsmoker. Extracting pivotal points from the history and physical exam enables the clinician to focus a broad differential diagnosis to a more limited set of diagnoses pertinent to that particular patient. The prerequisite for being able to construct an accurate problem representation is knowledge of the pivotal points for specific clinical problems.

Step 3: Develop a Complete, Framed Differential Diagnosis

The process for developing a differential diagnosis will be discussed later in this chapter; subsequent chapters will present comprehensive, framed differential diagnoses specific for each problem discussed.

Step 4: Prioritize the Differential Diagnosis

Not all diagnoses in a given differential are equally likely, or equally important. In order to effectively select diagnostic tests and therapies, it is necessary to select a “leading hypothesis,” a “must not miss” hypothesis, and other “active alternative hypotheses” (see full discussion later). The prerequisites for this step are knowledge of pivotal points; typical or “textbook” presentations of disease; the variability of disease presentation; and which diseases are life-threatening, very common, or easily treatable. It is also necessary to know how to estimate pretest probability, and which history, physical, or laboratory findings are so specific for a disease they are diagnostic; in other words, such findings are “fingerprints” for the disease.

Step 5: Test Your Hypothesis

Sometimes you are certain about the diagnosis based on the initial data ...

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