Beyond surveillance: a role for respondent-driven sampling in implementation science.

We are now in the fourth decade of the human immunodeficiency virus (HIV) pandemic. Several novel prevention tools have been identified, and prevalence and incidence have declined in many settings. A remaining challenge is the delivery of preventive interventions to hard-to-reach populations, including men who have sex with men and injection drug users. Leaders in the field of HIV have called for a new focus on implementation science, which requires a shift in thinking from individual randomized controlled trials to cluster-randomized trials. Multiple challenges need to be addressed in the conduct of cluster-randomized trials, including: 1) generalizability of the study population to the target population, 2) potential contamination through overlap/exchange of members of control and intervention clusters, and 3) evaluation of effectiveness at multiple levels of influence. To address these key challenges, we propose a novel application of respondent-driven sampling-a chain-referral strategy commonly used for surveillance-in the recruitment of participants for the evaluation of a cluster-randomized trial of a community intervention. We illustrate this application with an empirical example of a cluster-randomized trial that is currently under way to assess the effectiveness of men's wellness centers in improving utilization of HIV counseling and testing among men who have sex with men in India.