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Background  Many randomized controlled trials fail to reach their target sample size. When coupled with the omission and underrepresentation of disadvantaged groups in randomized controlled trials, many trials fail to obtain data that accurately represents the true diversity of their target population. Policies and practices have been implemented to increase representation of disadvantaged groups in many randomized controlled trials, with some trials specifically targeting such groups. To our knowledge, no systematic review has quantified the enrollment metrics and effectiveness of inclusion and retention strategies in randomized controlled trials focused on disadvantaged populations specifically.    Methods  We will conduct a systematic search across EMBASE, MEDLINE, Web of Science, and CINAHL as well as grey literature, conference proceedings, research monographs, and Google Scholar from inception onwards. We will include randomized controlled trials where at least 50% of enrolled participants are considered to be disadvantaged, as per the RCT authors’ definition and in line with our inclusion criteria. Two independent researchers per article will conduct preliminary title and abstract screening, subsequent full text review, and data extraction for the selected trials, with a third reviewer available to resolve conflicts. We will assess the quality of all included studies using specific criteria regarding data reporting, external validity, and internal validity. We will combine all selected studies and conduct a narrative synthesis to assess enrollment metrics. If there is sufficient homogeneity and sufficient trials comparing recruitment strategies within disadvantaged populations, we will conduct a random effects meta-analysis to evaluate the effectiveness of strategies designed to maximize the inclusion of disadvantaged populations in randomized controlled trials.    Discussion  The findings of this systematic review will establish baseline recruitment and enrollment metrics of trials targeting disadvantaged populations to elucidate the scope of the challenge of recruiting such populations. We hope that our findings will promote future research on the distinct barriers that may prevent disadvantaged populations from participating in health intervention research, will encourage more trials exploring effective, tailored recruitment strategies, and will establish a foundation to track future progress in the recruitment of disadvantaged populations.    Trial registrations  PROSPERO ID:  CRD42020152814Supplementary Information  The online version contains supplementary material available at 10.1186/s13643-021-01790-7.

Enrollment, retention, and strategies for including disadvantaged populations in randomized controlled trials: a systematic review protocol