What is iterative diagnosis?

The traditional model of diagnosis is one of initial collection of information in the history and examination, followed by deductive steps to reach a diagnosis. We suspect that most clinicians do not recognise or use this process.

A more realistic model was formulated by Elstein and Schwarz 25 years ago.1 It was called the hypothetico-deductive model, but we will call it the process of iterative diagnosis. This model recognises that clinical reasoning usually involves the clinician generating one or more possible hypotheses early on in the consultation (often, but not always, by pattern recognition2) and then recurrently—iteratively—testing these. Clinicians use many such shortcuts (heuristics) in clinical reasoning. This is not a fault: the shortcuts are typically correct and allow them to arrive at a working diagnosis with the minimum of delay, while avoiding excessive testing and anxiety. Exhaustive data collection without hypotheses—the medical student’s history and examination—usually does not improve diagnostic accuracy and may make it worse.

The initial steps in the process of making a diagnosis are therefore often non-analytical or intuitive.2 The initial hypothesis (the limited list of possible diagnoses) is often formulated before much data collection has occurred—from the “eyeball” impression as the patient walks in or as he or she is speaking.3 The process of testing of the hypothesis then proceeds by careful and systematic gathering of data and weighing the elicited information against the mental rules that are referred to in the literature as analytical reasoning.

Research in clinical reasoning is moving to a consensus that both analytic and non-analytical processes operate simultaneously in problem solving and that the clinician relies to a …