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

To provide an evidence base of estimates of the prevalence of problem drug use in inner London.

Re-analysis of three capture-recapture studies using subjects aged 15-49 years, that aim to estimate the hidden population from analysing the overlaps between three data sources.

Newham (1995) Camden and Islington (C&I) (1993/4) and Lambeth, Southwark and Lewisham (LSL) (1992).

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 court records. In LSL opiate users were analysed separately. The studies identified 1832 individuals in LSL, 543 in Newham, and 1321 in C&I.

Poisson models were fitted to the data testing different interactions between the data sources representing potential dependencies. The simplest model was selected on the basis of its AIC score and log-likelihood ratio tests.

The number of hidden problem drug users were estimated to be 12,500 (95% CI 9600-16,100) in LSL with 4400 (3200-6100) opiate users; 7000 (5000-10,000) in C&I and 3800 (2000-7200) in Newham. The prevalence of problem 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.

Despite the inherent problems with capture-recapture methods, our three studies establish an evidence base for estimates of problem drug use in London. It is important that a larger study is carried out in London.