Using Capture-Recapture Methods to Estimate the Population of People Who Inject Drugs in Washington, DC.

No current estimates exist for the size of the population of people who inject drugs (PWID) in the District of Columbia (DC). The WHO/UNAIDS Guidelines on Estimating the Size of Populations Most at Risk to HIV was used as the methodological framework to estimate the DC PWID population. The capture phase recruited harm reduction agency clients; the recapture phase recruited community-based PWID. The 951 participants were predominantly Black (83.9 %), male (69.8 %), and 40+ years of age (68.2 %). Approximately 50.3 % reported injecting drugs in the past 30 days. We estimate approximately 8829 (95 % CI 4899 and 12,759) PWID in DC. When adjusted for possible missed sub-populations of PWID, the estimate increases to 12,000; thus, the original estimate of approximately 9000 should be viewed in the context of the 95 % confidence interval. These evidence-based estimations should be used to determine program delivery needs and resource allocation for PWID in Washington, DC.