Strategies for increasing recruitment to randomised controlled trials: systematic review

Abstract

Background: Recruitment of participants into randomised controlled trials (RCTs) is critical for successful trial conduct. Although there have been two previous systematic reviews on related topics, the results (which identified specific interventions) were inconclusive and not generalizable. The aim of our study was to evaluate the relative effectiveness of recruitment strategies for participation in RCTs.

Methods and findings: A systematic review, using the PRISMA guideline for reporting of systematic reviews, that compared methods of recruiting individual study participants into an actual or mock RCT were included. We searched MEDLINE, Embase, The Cochrane Library, and reference lists of relevant studies. From over 16,000 titles or abstracts reviewed, 396 papers were retrieved and 37 studies were included, in which 18,812 of at least 59,354 people approached agreed to participate in a clinical RCT. Recruitment strategies were broadly divided into four groups: novel trial designs (eight studies), recruiter differences (eight studies), incentives (two studies), and provision of trial information (19 studies). Strategies that increased people's awareness of the health problem being studied (e.g., an interactive computer program [relative risk (RR) 1.48, 95% confidence interval (CI) 1.00-2.18], attendance at an education session [RR 1.14, 95% CI 1.01-1.28], addition of a health questionnaire [RR 1.37, 95% CI 1.14-1.66]), or a video about the health condition (RR 1.75, 95% CI 1.11-2.74), and also monetary incentives (RR1.39, 95% CI 1.13-1.64 to RR 1.53, 95% CI 1.28-1.84) improved recruitment. Increasing patients' understanding of the trial process, recruiter differences, and various methods of randomisation and consent design did not show a difference in recruitment. Consent rates were also higher for nonblinded trial design, but differential loss to follow up between groups may jeopardise the study findings. The study's main limitation was the necessity of modifying the search strategy with subsequent search updates because of changes in MEDLINE definitions. The abstracts of previous versions of this systematic review were published in 2002 and 2007.

Conclusion: Recruitment strategies that focus on increasing potential participants' awareness of the health problem being studied, its potential impact on their health, and their engagement in the learning process appeared to increase recruitment to clinical studies. Further trials of recruitment strategies that target engaging participants to increase their awareness of the health problems being studied and the potential impact on their health may confirm this hypothesis. Please see later in the article for the Editors' Summary.

Figure 1. Literature search.

Figure 2. Consent rate for RCTs.

Figure 3. Consent rates for novel trial designs.

RR, intervention recruitment strategy/standard recruitment strategy. Used total number/number of intervention strategies to calculate RR, so that the number of patients on standard strategies were not overrepresented; S, random assignment for participants, standard care for nonparticipants; 2, patients are told physician believes the experimental drug may be superior. Increased chance of receiving the experimental drug after consenting; 3, patients are told that they are allowed to increase or decrease their chance of receiving the new experimental drug after consenting; 4, experimental drug for participants, standard care for nonparticipants; 5, standard drug for participants, experimental drug for nonparticipants; 6, random assignment for participants, choice of either treatments for nonparticipants.

Figure 4. Consent rates for recruiter differences.

RR, intervention recruitment strategy/standard recruitment strategy.

Figure 5. Consent rates for incentives.

RR, intervention recruitment strategy/standard recruitment strategy. Used total number/number of intervention strategies to calculate RR, so that the number of patients on standard strategies were not overrepresented; S, random assignment for participants, standard care for nonparticipants; 1, small incentives (US$2 prepaid cash incentive); 2, larger incentive (US$15) contingent on response; 3, US$200 prize draw.

Figure 6. Consent rates for methods of providing information.

RR, intervention recruitment strategy/standard recruitment strategy. Used total number/number of intervention strategies to calculate RR, so that the number of patients on standard strategies were not overrepresented; S, standard informed consent; B, bulk mailing; 1, enhanced recruitment letter and screening by African American interviewer; 2, enhanced recruitment letter, screening by African American interviewer and baseline information collected via telephone interview; 3, enhanced recruitment letter, screening by African American interviewer and church-based project sessions; 4, bulk mailing with letter; 5, first-class mailing; 6, first-class mailing with letter.