A systematic review of economic evaluations alongside studies within a trial (SWATs) for improving recruitment and retention in randomised controlled trials

To review the cost-effectiveness of strategies to improve participant recruitment and retention in randomised controlled trials. All included studies from the latest Cochrane recruitment and retention reviews were considered. To identify articles published since the Cochrane reviews, electronic databases were searched until March 2021. Hand searching of conference databases and journals was also undertaken. The inclusion criteria included Studies within a Trial (SWATs). The main outcome was the incremental cost-effectiveness ratio (ICER). Quality assessment of papers used the Cochrane risk of bias 1 tool. The CRD guidance was used to assess the quality of economic evaluation. Random-effect meta-analyses were undertaken. The GRADE certainty of evidence was applied for each strategy, and Trial Forge Guidance 2 was used for strategies included in meta-analyses to evaluate the uncertainty of the findings. Cost-effectiveness ranks summarise the cost-effectiveness of all strategies. We identified 6569 records and included 29 SWATs (earliest conducted in 1999 and latest in 2021) including more than 35,800 participants. There is no strategy we would recommend trial teams and researchers adopt with complete statistical certainty. Recruitment strategies which could be cost-effective include financial incentives, trial-branded pens, telephone reminders and pre-notification leaflets. Retention strategies which could be cost-effective include vouchers and trial-branded pens. Future SWATs should replicate existing recruitment and retention strategies, rather than evaluate novel ones. We recommend that economic evaluations be carried out alongside all future SWATs, costs and benefits be recorded transparently, and the cost-effectiveness of existing recruitment or retention strategies be evaluated.