Capture-recapture estimates of the local prevalence of problem drug use in six French cities (2009)

BACKGROUND: General population surveys do not provide reliable estimates of problem drug users, which are essential for drug use prevention and treatment policies. Indirect estimation methods, especially capture-recapture method, are preferred. Problem drug use receives now a growing interest on a European level. Very few studies using this method have been carried out in France especially on a multicentric level.
METHOD: Six three-sample capture-recapture analyses were carried out in urban areas of Lille, Lyon, Marseille, Metz, Rennes, Toulouse. Data were collected in 2006 from different sources including treatment data, harm reduction and social data and law enforcement data. Users of opiates, cocaine/crack, stimulants and/or hallucinogens aged 15-64 were identified. Log-linear models were fitted to the data in order to test different interactions between the data sources representing potential dependencies. The simplest model was selected on the basis of its Akaike Information Criterion (AIC) and log-likelihood ratio tests. Confidence intervals were estimated using the likelihood interval method.
RESULTS: The total number of problem drug users was estimated to be 7900 (95% CI 6300-10 200) in Lille, 8400 (6300-11 800) in Lyon, 5600 (4200-7700) in Marseille, 2300 (1700-3200) in Metz, 1500 (1100-2300) in Rennes and 5400 (4300-6900) in Toulouse (estimates are rounded to nearest 100). These figures correspond to the following prevalence rates of the population aged 15-64 years: 10.8 per thousand (8.6-14.0) in Lille, 10.6 (8.0-15.0) in Lyon, 10.3 (7.7-14.2) in Marseille, 10.8 (8.0-15.0) in Metz, 7.6 (5.6-11.7) in Rennes, 10.1 (8.0-12.9) in Toulouse.
CONCLUSIONS: Although the confidence intervals are wide, the method provides valuable information on the extent of problem drug use. (Authors abstract)