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.
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