Patients' understanding of medical risks: implications for genetic counseling

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Abstract
Objective: To assess patients’ ability to compare magnitudes of Down syndrome risk at maternal ages of 35 and 40 years, expressed as rates or as proportions. Methods: We used a self-administered, anonymous questionnaire that posed the same comparison in two different formats: 2.6 versus 8.9 per 1000 women (rates) and one in 384 versus one in 112 women (proportions). The study setting included several university-affiliated obstetrics and gynecology outpatient clinics in San Francisco, California. A total of 633 women, whose primary languages were English, Spanish, or Chinese, participated. The main outcome measure was correct identification of the larger of two risks. Results: Women were more successful with rates (463 of 633 respondents, 73%) than with proportions (353 of 633 respondents, 56%). A paired analysis, in which each woman served as her own control, found risk assessment to be significantly better with rates than with proportions (P < .001). Women with little formal education had difficulty understanding risks framed either way. Conclusion: The traditional use of proportions to express risk in genetic counseling lacks scientific basis. Rates were easier to understand than proportions, regardless of respondents’ age, language, and education.

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