Assessing the assumptions of respondent-driven sampling in the national HIV Behavioral Surveillance System among injecting drug users

Several assumptions determine whether respondent-driven sampling (RDS) is an appropriate sampling method to use with a particular group, including the population being recruited must know one another as members of the group (i.e., injection drug users [IDUs] must know each other as IDUs) and be networked and that the sample size is small relative to the overall size of the group. To assess these three assumptions, we analyzed city-specific data collected using RDS through the US National HIV Behavioral Surveillance System among IDUs in 23 cities. Overall, 5% of non-seed participants reported that their recruiter was "a stranger." 20 cities with multiple field sites had ≥1 cross-recruitment, a proxy for linked networks. Sample sizes were small in relation to the IDU population size (median = 2.3%; range: 0.6%- 8.0%). Researchers must evaluate whether these three assumptions were met to justify the basis for using RDS to sample specific populations.