Approaches to Differential Diagnosis

Abstract

Medical students should take an active approach to differential diagnosis, with hypothesis testing in real time as they perform the history and physical exam. In this chapter, a number of diagnostic theories and principles are discussed with case examples, including Occam’s Razor, Hickam’s Dictum, the Law of Sigma, the post hoc ergo propter hoc fallacy, and heuristics. Causes of diagnostic error and bias are reviewed, with cases that illustrate premature closure, anchoring bias, and the framing effect. For medical students, the key findings approach is a simple, stepwise process that leads to a prioritized differential diagnosis and a rational testing strategy. In complex cases, using the key findings approach to create a diagnostic table can be helpful, as shown in the case of a patient with migratory polyarthritis and fever. Diagnosis is probabilistic, absolute certainty is rare, and students must learn to rank the probabilities and come up with a treatment plan based on the best available evidence.