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Clinical decision making is laced with uncertainty, and efficiently sorting out an underlying diagnosis as the cause of a patient's ailment can be difficult. Novice clinicians may attempt a stepwise assessment of all possible explanations for a patient's concerns until finally arriving at the diagnosis. More seasoned clinicians make use of extensive clinical experience, pattern recognition, and a range of approaches to save patients from the potential delays, risks, and costs of inefficient diagnostic strategies. The wisest clinicians seek out best evidence from clinical research and use it to complement their clinical experiences.1 In this chapter, we introduce the concepts of differential diagnosis, pretest probabilities, and test performance characteristics to bolster our abilities to make diagnoses.

Patients come to us for help with their concerns. As clinicians, we seek out additional information about the chief complaint and synthesize key findings into a clinical problem. To accomplish this efficiently, we must be well versed in the typical manifestations of various diseases, the frequency of the potential underlying causes, and the value of specific historical features in arriving at the most likely diagnosis.2–5 The purpose of this book is to provide this kind of information.

If a patient's complaint is headache, it helps to know which diseases cause headache, their principal clinical manifestations, and the relative frequency of each possibility ordered from the most common to the rarest. We generate further questioning based on how powerful or predictive certain findings are to either increase or decrease our suspicion for a specific diagnosis.

This process is known as differential diagnosis, in which a clinician uses prior knowledge alongside unique clinical findings from the patient, to prioritize the list of possibilities into a leading suspicion or working diagnosis, a few potentially active alternatives, and other possibilities that do not merit further consideration because they are not sufficiently likely or serious.4,6,7 Rather than memorizing an exhaustive list, differential diagnosis is a dynamic process in which clinicians use certain information or test results to modify their suspicion for a given disorder until the diagnosis can be identified with confidence.

Key clinical findings commonly include epidemiologic factors such as age and gender. For instance, your suspicion of migraine as the cause of a recent-onset headache in a 25-year-old woman is much higher than for a 65-year-old man with the same symptom.8 Experienced clinicians use each piece of additional epidemiologic and clinical information to continually narrow the diagnostic possibilities. Think of this process as moving through a funnel. Initial considerations are broad, but as the history progresses, a smaller number of plausible diagnoses remains.

Consider the use of specific questions during focused history taking as analogous to performing diagnostic tests. Each question should either increase or decrease the likelihood of the suspected underlying disorder. Returning to our headache example, the presence of nausea, photophobia, and throbbing pain each increase the chance that migraine is ...

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