Participant recruitment, baseline characteristics and at-home-measurements of cardiometabolic risk markers: insights from the Supreme Nudge parallel cluster-randomised controlled supermarket trial
Authors: Stuber Josine M. , van Hoek Beryl A. C. E. , Vos Anne L. , Smit Edith G. , Lakerveld Jeroen , Mackenbach Joreintje D. , Beulens Joline W. J. ,   on behalf of the SUPREME NUDGE consortium, Hoenink Jody C. , Rutters Femke , Waterlander Wilma E. , de Ridder Denise T. D. , Gillebaart Marleen , Blom Stephanie , de Boer Femke E. , de Bruijn Gert-Jan , Klein Michel C. A. , Broerse Jacqueline E. W. , Schuitmaker-Warnaar Tjerk-Jan , Middel Cédric N. H. , van der Schouw Yvonne T. , Sluijs Ivonne , Harbers Marjolein C. , Velema Elizabeth 
Published: March 2023

Abstract
Background
Recruiting participants for lifestyle programmes is known to be challenging. Insights into recruitment strategies, enrolment rates and costs are valuable but rarely reported. We provide insight into the costs and results of used recruitment strategies, baseline characteristics and feasibility of at-home cardiometabolic measurements as part of the Supreme Nudge trial investigating healthy lifestyle behaviours. This trial was conducted during the COVID-19 pandemic, requiring a largely remote data collection approach. Potential sociodemographic differences were explored between participants recruited through various strategies and for at-home measurement completion rates.

Methods
Participants were recruited from socially disadvantaged areas around participating study supermarkets (n = 12 supermarkets) across the Netherlands, aged 30–80 years, and regular shoppers of the participating supermarkets. Recruitment strategies, costs and yields were logged, together with completion rates of at-home measurements of cardiometabolic markers. Descriptive statistics are reported on recruitment yield per used method and baseline characteristics. We used linear and logistic multilevel models to assess the potential sociodemographic differences.

Results
Of 783 recruited, 602 were eligible to participate, and 421 completed informed consent. Most included participants were recruited via letters/flyers at home (75%), but this strategy was very costly per included participant (89 Euros). Of paid strategies, supermarket flyers were the cheapest (12 Euros) and the least time-invasive (< 1 h). Participants who completed baseline measurements (n = 391) were on average 57.6 (SD 11.0) years, 72% were female and 41% had high educational attainment, and they often completed the at-home measurements successfully (lipid profile 88%, HbA1c 94%, waist circumference 99%). Multilevel models suggested that males tended to be recruited more often via word-of-mouth (ORfemales 0.51 (95%CI 0.22; 1.21)). Those who failed the first attempt at completing the at-home blood measurement were older (β 3.89 years (95% CI 1.28; 6.49), whilst the non-completers of the HbA1c (β − 8.92 years (95% CI − 13.62; − 4.28)) and LDL (β − 3.19 years (95% CI − 6.53; 0.09)) were younger.

Conclusions
Supermarket flyers were the most cost-effective paid strategy, whereas mailings to home addresses recruited the most participants but were very costly. At-home cardiometabolic measurements were feasible and may be useful in geographically widespread groups or when face to face contact is not possible.

Trial registration
Dutch Trial Register ID NL7064, 30 May 2018, https://trialsearch.who.int/Trial2.aspx?TrialID=NTR7302