Patients' understanding of medical risks: Implications for genetic counseling

Object ive: To assess patients' ability to compare magnitudes of Down syndr ome risk at maternal ages of 35 and 40 years, expressed as rates or as prop ortions. Methods: We used a self-administered, anonymous questionnaire that posed th e 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 sett ing included several university-affiliated obstetrics and gynecology outpat ient clinics in San Francisco, California. A total of 633 women, whose prim ary languages were English, Spanish, or Chinese, participated. The main out come 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, i n which each woman served as her own control, found risk assessment to be s ignificantly better with rates than with proportions (P < .001). Women with little formal education had difficulty understanding risks framed either w ay. Conclusion: The traditional use of proportions to express risk in genetic c ounseling lacks scientific basis. Rates were easier to understand than prop ortions, regardless of respondents' age, language, and education. (Obstet G ynecol 1999; 93:910-4. (C) 1999 by The American College of Obstetricians an d Gynecologists.)