The Accuracy of Patients' Judgments of Disease Probability and Test Sensitivity and Specificity

Abstract

Background. To find out what patients know concerning diagnostic uncertainties before they communicate with a doctor, we studied their understanding of the characteristics of diagnostic tests for six common conditions. We compared the accuracy of patients' estimates of disease probabilities and diagnostic test characteristics for diseases with which they did or did not have prior experience. Methods. To measure patients' understanding of uncertainty of diagnostic test results, questionnaires describing diseases were given to patients in clinic waiting rooms. For each of six diseases, a two page questionnaire (1) presented a case history of disease and diagnostic test; (2) asked respondent to estimate (a) probability the case patient has the suspected disease, (b) sensitivity of test, (c) specificity of test, and (d) probability of disease given positive test; and (3) asked whether patient or close friend or family member had ever been thought to have this disease. Results. In clinic waiting room, 184 patients responded for at least one disease. Although patients judged the disease probabilities to be higher after a positive diagnostic test, each of their four judgments was essentially the same for all diseases, including those with high and low prior probabilities, and with accurate and inaccurate tests. Past experience with the disease was associated with only a minimal increase in the accuracy of patient knowledge. Conclusions. Patient ignorance of the uncertainties of diseases they might encounter demonstrates the need for patient education when a disease is suspected. Lack of relation between accuracy and experience suggests this need is not being effectively met.