Characteristics and health service utilization in two samples of young injection drug users recruited using direct and referral methods in San Francisco, California.

Targeted sampling (TS) and respondent-driven sampling methods (RDS) are commonly used methods to sample ‘hard-toreach’ populations, like injection drug users (IDU). There are notable differences between each: TS uses street-based and community based outreach, informed by ethnographic data to locate and recruit a target population [1], but lacks statistical methods to correct for potential biases. RDS is a modified snowball sampling method that utilizes a dualincentive chain referral system through controlled coupons where waves of recruitment are used to penetrate a population until sample size and stability are achieved; wherein specialized estimators are used to adjust for biases in the probability of inclusion and recruitment patterns [2]. Neither method is guaranteed nor absolute in their attempt to collect generalizable data, but both are motivated by the need for sampling methods that can be used to design and improve disease surveillance and public health interventions [1-4]. Kral et al. [5] recently compared characteristics and self-reported service utilization of adult IDU in San Francisco from two studies that used TS and RDS, respectively. They found that there were similarities across all demographics except the African American race as well as age. Risk behaviors were also similarly distributed in the two samples, but a lower proportion of the RDS sample accessed drug treatment, syringe exchange programs (SEP), and also had lower self-reported prevalence of hepatitis C virus (HCV) infection. This comparison however, was limited, as few young adult IDU were included; the majority (≥ 85%) were 36 years or older. While San Francisco’s general population is mostly white (48.5%) and Asian (34.2%) [6], surveillance studies depict an IDU population comprised of mostly non-Hispanic Whites (40.5%) and African Americans (32.6%) individuals in San Francisco [7]; however, the demographics of national samples depict an IDU population comprised of mostly African Americans (49%) and fewer non-Hispanic Whites (25%) [8].