Evaluation of a holiday card retention strategy: a study within a trial (SWAT) embedded in a blood pressure reduction trial

Introduction Participant retention is essential to minimize bias, preserve statistical power, and strengthen the validity of clinical trial findings. Although retention strategies are widely used, many lack rigorous evaluation. We evaluated whether sending a holiday card, compared with usual follow-up procedure, improves follow-up completion among clinical trial participants recruited from a resource-constrained emergency department. Methods We conducted a Study Within a Trial (SWAT) embedded within Reach Out, a 2 × 2 × 2 randomized trial of a mobile health-based behavioral intervention designed to reduce blood pressure among individuals with elevated blood pressure identified from a resource-constrained emergency department. All participants received standard retention strategies as part of the parent trial, with participants randomized to the SWAT intervention group additionally receiving a holiday card. The primary outcome was completion of the next available outcome assessment (6- or 12-month assessment). We estimated absolute differences with 95% confidence intervals in outcome assessment completion between participants who received a holiday card and those who did not, and then stratified by age, sex, and race. Results Among Reach Out participants (N = 395), 198 were randomized to receive a holiday card and 197 to receive no card. Outcome assessment completion did not differ between participants who received a holiday card and those who did not (54.5% vs 58.4%; p = 0.43), and there was no evidence of effect modification by age, sex, or race. Conclusion In this low-resource setting, receipt of a holiday card was not associated with higher outcome completion. These findings are consistent with prior studies and suggest that multicomponent interventions may be needed to increase retention, underscoring the value of pragmatic evaluations of retention interventions embedded within ongoing trials.