Respondent-driven sampling of injection drug users in two U.S.-Mexico border cities: recruitment dynamics and impact on estimates of HIV and syphilis prevalence

Abstract

Respondent-driven sampling (RDS), a chain referral sampling approach, is increasingly used to recruit participants from hard-to-reach populations, such as injection drug users (IDUs). Using RDS, we recruited IDUs in Tijuana and Ciudad (Cd.) Juárez, two Mexican cities bordering San Diego, CA and El Paso, TX, respectively, and compared recruitment dynamics, reported network size, and estimates of HIV and syphilis prevalence. Between February and April 2005, we used RDS to recruit IDUs in Tijuana (15 seeds, 207 recruits) and Cd. Juárez (9 seeds, 197 recruits), Mexico for a cross-sectional study of behavioral and contextual factors associated with HIV, HCV and syphilis infections. All subjects provided informed consent, an anonymous interview, and a venous blood sample for serologic testing of HIV, HCV, HBV (Cd. Juárez only) and syphilis antibody. Log-linear models were used to analyze the association between the state of the recruiter and that of the recruitee in the referral chains, and population estimates of the presence of syphilis antibody were obtained, correcting for biased sampling using RDS-based estimators. Sampling of the targeted 200 recruits per city was achieved rapidly (2 months in Tijuana, 2 weeks in Cd. Juárez). After excluding seeds and missing data, the sample prevalence of HCV, HIV and syphilis were 96.6, 1.9 and 13.5% respectively in Tijuana, and 95.3, 4.1, and 2.7% respectively in Cd. Juárez (where HBV prevalence was 84.7%). Syphilis cases were clustered in recruitment trees. RDS-corrected estimates of syphilis antibody prevalence ranged from 12.8 to 26.8% in Tijuana and from 2.9 to 15.6% in Ciudad Juárez, depending on how recruitment patterns were modeled, and assumptions about how network size affected an individual's probability of being included in the sample. RDS was an effective method to rapidly recruit IDUs in these cities. Although the frequency of HIV was low, syphilis prevalence was high, particularly in Tijuana. RDS-corrected estimates of syphilis prevalence were sensitive to model assumptions, suggesting that further validation of RDS is necessary.

Figure 1

Summary of the dynamics of recruitment, by city. (a) The cumulative number of recruits over time. (b) The interval between the interview of the recruiter and that of their recruitee (omitting those individuals who did not recruit). (c) The number of recruits in each recruitment wave from the seed. (d) The number of recruits per recruiter (excluding individuals who were not given any coupons). (e) The number of recruits from each seed. (f) The relationship between recruiter and recruitee.

Figure 2

Recruitment networks (strictly speaking, a forest of recruitment trees) for the RDS based samples of IDUs in (a) Tijuana and (b) Ciudad Juárez. Seeds are shown at the top of the figure, and arrows indicate the direction of recruitment. Syphilis serostatus is shown by shading: black- syphilis antibody positive, white- syphilis antibody negative, gray- missing data. The gender of participants is indicated by the shape of the symbol: square for female and circle for male. The size of the symbol is related to the reported network size: the larger the symbol, the larger the network size. Symbols marked with an ‘×’ denote individuals who were given coupons, but did not recruit.

Figure 3

Summary of the network size distribution. (a) Scatterplot of the number of IDUs known by name or street name between recruiter and recruitee. (b) Scatterplot of the number of injecting partners between recruiter and recruitee. (c) Boxplot of the number of IDUs known by sex (M male, F female) and by syphilis antibody status (+ positive, − negative). (d) Boxplot of the number of injecting partners by sex (M male, F female) and by syphilis antibody status (+ positive, − negative). (e) Cumulative distribution of the number of IDUs known before (solid line) and after (dashed line) adjusting for biased sampling of individuals.