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Drug Alcohol Depend. 2014 Sep 1;142:174-80. doi: 10.1016/j.drugalcdep.2014.06.022. Epub 2014 Jun 24.

Estimating the size of the population of persons who inject drugs in the island of Montréal, Canada, using a six-source capture-recapture model.

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Direction de santé publique de l'Agence de la santé et des services sociaux de Montréal, 1301 rue Sherbrooke est, Montréal, Québec, Canada H2L 1M3. Electronic address:
Department of Biostatistics and Epidemiology, Faculty of Health & Environmental Sciences, Auckland University of Technology, Private bag 92006, Auckland 1142, New Zealand; Health Intelligence and Informatics, Ko Awatea, Counties Manukau District Health Board, Private bag 93311, Auckland 1640, New Zealand.
Direction de santé publique de l'Agence de la santé et des services sociaux de Montréal, 1301 rue Sherbrooke est, Montréal, Québec, Canada H2L 1M3.
Centre de recherche, Centre hospitalier de l'Université de Montréal (CRCHUM), 850 Saint-Denis, Montréal, Québec, Canada H2X 0A9.
Faculté de médecine et des sciences de la santé, Université de Sherbrooke, Campus Longueuil, 150 place Charles-Le Moyne, Longueuil, Québec, Canada J4K 0A8.
Service de Médecine des toxicomanies, CHUM, 1058 St-Denis, Montreal, Québec, Canada H2X 3J4.
Surveillance and Epidemiology Division, Centre for Communicable Diseases and Infection Control Public Health Agency of Canada, Room 3305, LCDC Building, 100 Eglantine Driveway, Tunney's Pasture A/L: 0602B, Ottawa, Canada K1A 0K9.



To plan and implement services to persons who inject drugs (PWID), knowing their number is essential. For the island of Montréal, Canada, the only estimate, of 11,700 PWID, was obtained in 1996 through a capture-recapture method. Thirteen years later, this study was undertaken to produce a new estimate.


PWID were defined as individuals aged 14-65 years, having injected recently and living on the island of Montréal. The study period was 07/01/2009 to 06/30/2010. An estimate was produced using a six-source capture-recapture log-linear regression method. The data sources were two epidemiological studies and four drug dependence treatment centres. Model selection was conducted in two steps, the first focusing on interactions between sources and the second, on age group and gender as covariates and as modulators of interactions.


A total of 1480 PWID were identified in the six capture sources. They corresponded to 1132 different individuals. Based on the best-fitting model, which included age group and sex as covariates and six two-source interactions (some modulated by age), the estimated population was 3910 PWID (95% confidence intervals (CI): 3180-4900) which represents a prevalence of 2.8 (95% CI: 2.3-3.5) PWID per 1000 persons aged 14-65 years.


The 2009-2010 estimate represents a two-third reduction compared to the one for 1996. The multisource capture-recapture method is useful to produce estimates of the size of the PWID population. It is of particular interest when conducted at regular intervals thus allowing for close monitoring of the injection phenomenon.


Canada; Capture–recapture; Injection drug use; Log-linear model

[Indexed for MEDLINE]

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