Results: 45 trials with over 43 000 participants were
included. Some interventions were effective in increasing
recruitment: telephone reminders to non-respondents (risk
ratio (RR) 1.66, 95% CI 1.03 to 2.46; two studies, 1058
participants), use of opt-out rather than opt-in procedures
for contacting potential participants (RR 1.39, 95% CI
1.06 to 1.84; one study, 152 participants) and open
designs where participants know which treatment they are
receiving in the trial (RR 1.22, 95% CI 1.09 to 1.36; two
studies, 4833 participants). However, the effect of many
other strategies is less clear, including the use of video to
provide trial information and interventions aimed at
recruiters.
Conclusions: There are promising strategies for
increasing recruitment to trials, but some methods,
such as open-trial designs and opt-out strategies, must
be considered carefully as their use may also present
methodological or ethical challenges. Questions remain
as to the applicability of results originating from
hypothetical trials, including those relating to the use
of monetary incentives, and there is a clear knowledge
gap with regard to effective strategies aimed at
recruiters.