Written informed consent for participation in a study and reduction in consent rate.

BACKGROUND The association between the method of obtaining informed consent and the consent rate in a cohort study, as well as the differences between consenters and non-consenters with regard to blood-sample donation are unclear. METHODS We measured the consent rates among 64-year-old residents who underwent medical checkups in a city for a cohort study consisting of a questionnaire survey and blood-sample donation and determined the influence of different approaches to informed consent and the participants' characteristics on the consent rates. RESULTS Of 3,098 residents who underwent medical checkups over 10 years, 99.2% responded to the questionnaire survey, and 92.5% agreed to blood-sample donation. The consent rate for blood-sample donation after obtaining individual written informed consent was lower than that observed with the general-announcement approach. Differences in the consent rates for participation in the questionnaire study were, however, negligible. A higher percentage of men than women consented to donate blood samples. After adjustments for gender, it was observed that individuals with a history of hypertension and those without depression consented to blood-sample donation significantly more frequently. CONCLUSION The consent rate for blood-sample donation to the study decreased when the opt-in approach with written consent was used. This decrease may introduce consent bias, and the method of obtaining informed consent should be revised.