Recruiting patients to medical research: double blind randomised trial of “opt-in” versus “opt-out” strategies

OBJECTIVE: To evaluate the effect of opt-in compared with opt-out recruitment strategies on response rate and selection bias. DESIGN: Double blind randomised controlled trial. SETTING: Two general practices in England. PARTICIPANTS: 510 patients with angina. INTERVENTION: Patients were randomly allocated to an opt-in (asked to actively signal willingness to participate in research) or opt-out (contacted repeatedly unless they signalled unwillingness to participate) approach for recruitment to an observational prognostic study of patients with angina. MAIN OUTCOME MEASURES: Recruitment rate and clinical characteristics of patients. RESULTS: The recruitment rate, defined by clinic attendance, was 38% (96/252) in the opt-in arm and 50% (128/258) in the opt-out arm (P = 0.014). Once an appointment had been made, non-attendance at the clinic was similar (20% opt-in arm v 17% opt-out arm; P = 0.86). Patients in the opt-in arm had fewer risk factors (44% v 60%; P = 0.053), less treatment for angina (69% v 82%; P = 0.010), and less functional impairment (9% v 20%; P = 0.023) than patients in the opt-out arm. CONCLUSIONS: The opt-in approach to participant recruitment, increasingly required by ethics committees, resulted in lower response rates and a biased sample. We propose that the opt-out approach should be the default recruitment strategy for studies with low risk to participants.