The TS study was the 37th semiannual cross-section of the Urban Health Study (UHS), a study of IDUs in San Francisco. Watters and Biernacki,9 (link) the developers of TS, first implemented the method in the first UHS cross-section in 1986. The procedures and results from this study have been published extensively over the past two decades.29 (link)–31 (link) Briefly, the five steps involved in developing and refining the TS plan are as follows: (1) Collect indicator data from sources including drug treatment admissions data, IDU HIV/AIDS case data from the San Francisco Department of Public Health (SFDPH), data from Citywide HIV testing sites, and narcotics arrest data from the San Francisco police departments. (2) Collect ethnographic data through block-by-block “walk-throughs,” which were conducted at different times of day and night. Ethnographic data collection also included discussions with key informants regarding their observations on the prevalence and type of drug use in various communities. (3) Develop TS plan by estimating the size and demographic characteristics of the IDU population in each area and set recruitment targets. (4) Conduct outreach to recruit IDUs to community field sites in the neighborhoods with the highest concentration of IDUs per secondary data (marked by stars in Fig. 1b). (5) Refine and revise targeted sample after monthly assessments of how well the recruitment targets were met.

Distribution of study recruits by zip code. a RDS. b Targeted sampling.

Eligibility criteria included the following: (1) reported injecting illicit drugs within the past 30 days, (2) had visible sign of injection (“tracks”), (3) were at least 18 years of age at the time of interview, and (4) were able to speak English or Spanish. Participants from previous serial cross-sections did not receive any special recruitment contact, but they were automatically eligible for future cross-sections (6 months apart), even if they had switched to noninjecting methods or had quit using drugs. Participants were assigned a unique ID code, and we checked their identification by asking five questions: sex, birth year, age, race/ethnicity, state of birth, and first two letters of mother's maiden name. This helped to determine which observations were duplicates. For this analysis, we only included active IDUs who reported injecting drugs in the previous 30 days.
After providing informed consent to this anonymous study, participants were interviewed in person by a trained interviewer in a private space using a structured questionnaire on a computer-assisted programmed interview (CAPI) using QDS software (QDS; NOVA Research Company, Bethesda, MD). They were paid $15 for contributing to the study. Questions covered demographic information, injecting and sexual risk behaviors, and utilization of health care, drug treatment, and HIV prevention programs. The questions pertained to the 6-month period preceding the interview date. Blood specimens were drawn following the interview to assess HIV status using enzyme immunoassay and Western blot assay, following standard laboratory methods. Participants were asked to return for HIV serology results in 2 weeks. They were offered HIV counseling, provided with referrals to medical and social services, and paid $15. Study protocols were approved by the University of California, San Francisco (UCSF) Committee on Human Research.