Capturing crack cocaine use: estimating the prevalence of crack cocaine use in London using capture-recapture with covariates.

AIM: To estimate the prevalence of crack cocaine use in 12 London Boroughs (and London as a whole).
SETTING: Twelve London Boroughs, 2000-01.
METHODS: (1) Covariate capture-recapture techniques applied to three data sources of subjects reporting crack cocaine use: specialist drug treatment (2905), arrest referral (1188) and accident and emergency and community survey (531); and (2) ratio-estimation multiplier, using an estimate of number of injecting drug users and proportion that use crack cocaine.
FINDINGS: After matching, 4117 individuals aged 15-44 were identified. The best-fitting model estimated 16 855 unobserved crack cocaine users, giving an overall estimate of approximately 21 000 [95% confidence interval (CI) 13 000-43,000] and a prevalence of 1.5% (95% CI 1.0-3.2%). Prevalence of crack cocaine use was 2.4% (95% CI 1.5-5.0%) among men and 0.7% (95% CI 0.5-1.0%) among women, and similar by age groups 15-29 and 30-44 years. Overall, approximately 11 900 (57%) of the estimated number of crack cocaine users were also opiate users. In London as a whole there may be 46,000 (1.3%) crack cocaine users aged 15-44 years, with 28 000 (1.9%) in inner London-four times higher than estimates from population surveys. Some corroboration was provided by the ratio-estimation method, which estimated 23 000 users in the 12 Boroughs.
CONCLUSIONS: Capture-recapture can be applied to crack cocaine and obtain better estimates than population surveys. The size of the crack cocaine-using population in London is large, although currently the majority are also opiate users. Given that half of current users are under 30 the problems associated with crack cocaine use are likely to increase in the future.