The accuracy of patients' judgments of disease probability and test sensitivity and specificity.

BACKGROUND: We studied patients' understanding of the characteristics of diagnostic tests for six common conditions to determine what patients know about diagnostic uncertainties before they communicate with a doctor. 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 the uncertainty of diagnostic test results, questionnaires describing diseases were given to patients in clinic waiting rooms. For each of six diseases, a 2-page questionnaire presented a case history of the disease and its diagnostic test, and asked respondents to estimate the probability that the case patient has the suspected disease, the sensitivity of test, the specificity of test, and the probability that the patient has the disease if the test result is positive. It also asked whether the patient, a close friend, or family member had ever been thought to have this disease.
RESULTS: One hundred eighty-four patients in the clinic waiting room 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: Patients' ignorance of the uncertainties of diseases demonstrates the need for patient education when a disease is suspected. The lack of relation between knowledge and experience suggests that this need is not being effectively met. To convey the rates or probabilities, and to help the patients understand what the probabilities are based on, a physician should speak in terms that patients can easily understand.