A systematic review of recruitment and retention within randomised controlled trials of adults with Down Syndrome

Background: Adults with Down Syndrome (DS) face significant health inequalities. Evidence from high-quality randomised controlled trials (RCTs) for interventions are lacking, and there is a perceived difficulty regarding recruiting and retaining participants with DS. Aims: This systematic review aimed to examine planned vs. actual recruitment and retention rates of RCTs involving adults with DS, summarising reported facilitators of and barriers to participation.Method: MEDLINE, PsycINFO, and EMBASE databases were searched systematically to retrieve RCTs of any intervention involving adults with DS =>16yrs, published to 07/12/2023, with a second search in April 2025. Searches were also conducted on trial registries, and reference lists reviewed. Trial design, planned and achieved sample size, retention rate, and recruitment and retention strategies were extracted 26/02/2024-18/03/2024 for the first search, and in August 2025 for the updated search (Prospero: CRD42023447126; registered: 04/09/2023). Results: After deduplication, 1,622 articles underwent title and abstract screening, with 89 full texts reviewed. Five additional papers were identified from reference lists. 1,785 adults with DS were randomised to 44 included RCTs. Most studies did not pre-register their protocol nor target sample size (n=16 reported target N; 36.36%) and typically recruited small samples: 13 recruited N ≥40 (29.55%; N range=8-337, median N=25, IQR 21.75). Attrition by primary outcome also varied widely (0-47%).Conclusions: There is a paucity of high-quality, large-scale RCTs involving adults with DS, and challenges related to participation rates (e.g., stringent exclusion criteria, consent challenges, and study burden). Strategies to overcome challenges in recruitment and retention to such trials are discussed.