Estimating the prevalence of problem drug use in inner London: a discussion of three capture-recapture studies

Aims. To provide art evidence base of estimates of the prevalence of proble m drug use in inner London. Design. Re-analysis of three capture-recapture studies using subjects aged 15-49 years, that aim to estimate the hidden po pulation from analysing the overlaps between three data sources. Setting, N ewham (1995) Camden and Islington (C&I) (1993/4) and Lambeth, Southwark and Lewisham (LSL) (1992). Participants. Each study collected data from three sources of problem drug users including: the Regional Drug Misuse Database, specialist drug agencies, HIV tests, social services, police arrests and c ourt records. In LSL opiate users were analysed separately. The studies ide ntified 1832 individuals in LSL, 543 in Newham, and 1321 in C&I. Measuremen ts. Poisson models were fitted to the data testing different interactions b etween the data sources representing potential dependencies. The simplest m odel was selected on the basis of its AIC score and log-likelihood ratio te sts. Findings. The number of hidden problem drug users were estimated to be 12 500 (95% CI 9600-16100) in LSL with 4400 (3200-6100) opiate users; 7000 (5000-10000) in C&I and 3800 (2000-7200) in Newham. The prevalence of prob lem drug use in those aged 15-49 was estimated to be 3.1% (2.5-3.9%) in LSL with 1.3% (1.0-1.6%) opiate users; and 3.6% (2.7-4.9%) and 3.3% (1.9-5.7%) in C&I and Newham, respectively. Conclusions. Despite the inherent problem s with capture-recapture methods, our three studies establish an evidence b ase for estimates of problem drug use in London. It is important that a lar ger study is carried out in London.