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Confirmation Bias

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The tendency to focus on evidence that supports a working hypothesis, such as a diagnosis in clinical medicine, rather than to look for evidence that refutes it or provides greater support to an alternative diagnosis. Suppose that a 65-year-old man with a past history of angina presents to the emergency department with acute onset of shortness of breath. The physician immediately considers the possibility of cardiac ischemia, so asks the patient if he has experienced any chest pain. The patient replies affirmatively. Because the physician perceives this answer as confirming his working diagnosis, he does not ask if the chest pain was pleuritic in nature, which would decrease the likelihood of an acute coronary syndrome and increase the likelihood of pulmonary embolism (a reasonable alternative diagnosis for acute shortness of breath accompanied by chest pain). The physician then orders an EKG and cardiac troponin. The EKG shows nonspecific ST changes and the troponin returns slightly elevated.

Of course, ordering an EKG and testing cardiac enzymes is appropriate in the work-up of acute shortness of breath, especially when it is accompanied by chest pain and in a patient with known angina. The problem is that these tests may be misleading, since positive results are consistent not only with acute coronary syndrome but also with pulmonary embolism. To avoid confirmation in this case, the physician might have obtained an arterial blood glass or a D-dimer level. Abnormal results for either of these tests would be relatively unlikely to occur in a patient with an acute coronary syndrome (unless complicated by pulmonary edema), but likely to occur with pulmonary embolism. These results could be followed up by more direct testing for pulmonary embolism (e.g., with a helical CT scan of the chest), whereas normal results would allow the clinician to proceed with greater confidence down the road of investigating and managing cardiac ischemia.

This vignette was presented as if information were sought in sequence. In many cases, especially in acute care medicine, clinicians have the results of numerous tests in hand when they first meet a patient. The results of these tests often do not all suggest the same diagnosis. The appeal of accentuating confirmatory test results and ignoring nonconfirmatory ones is that it minimizes cognitive dissonance.

A related cognitive trap that may accompany confirmation bias and compound the possibility of error is "anchoring bias"—the tendency to stick with one's first impressions, even in the face of significant disconfirming evidence.

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