Are retention strategies used in National Institute for Health and Care Research, Health Technology Assessment trials supported by evidence for their effectiveness? A systematic mapping review

Background and Aims Poor retention of trial participants is common and can result in significant methodological, statistical, ethical, and financial challenges. To improve trial efficiency, we aimed to assess the extent to which commonly used strategies to retain participants within trials are supported by evidence for their effectiveness. Method A systematic methodological review was carried out to identify commonly used retention strategies in National Institute for Health and Care Research (NIHR) Health Technology Assessment (HTA) trials (January 2020–June 2022). Strategies were then mapped to evidence for their effectiveness from the most recent Cochrane retention review (published 2021), and a future Study Within A Trial (SWAT) priority list was created. Results Amongst 80 trials, the most frequently reported retention strategies were: flexibility with data collection method/location (53%); participant diaries (38%); use of routine data (29%); PPI input (26%); telephone reminders for participants (26%); postal reminders for participants (25%); monitoring approaches (21%); offering flexibility with timing of data collection (20%); pre-paid return postage (18%); prioritising collection of key outcomes (15%); and participant newsletters (15%). Out of the 56 identified strategies, mostly no, very low or low evidence for their effectiveness was identified (64%; 14%; 13% respectively). Discussion and Conclusions Commonly used retention strategies are lacking good quality evidence for their effectiveness. The findings support the need for more SWATs and help identify priority areas for future SWAT research. These priorities could be used with other priority lists to inform future SWAT conduct.