... Introduction. Methods. Laure Vaissade 1, Stéphane Legleye 1,2. Capture recapture. Participating cities

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1 European Journal of Public Health, Vol. 19, No. 1, ß The Author Published by Oxford University Press on behalf of the European Public Health Association. All rights reserved. doi: /eurpub/ckn126 Advance Access published on 26 December Capture recapture estimates of the local prevalence of problem drug use in six French cities Laure Vaissade 1, Stéphane Legleye 1,2 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 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 ) in Lille, 8400 ( ) in Lyon, 5600 ( ) in Marseille, 2300 ( ) in Metz, 1500 ( ) in Rennes and 5400 ( ) in Toulouse (estimates are rounded to nearest 100). These figures correspond to the following prevalence rates of the population aged years: 10.8 per thousand ( ) in Lille, 10.6 ( ) in Lyon, 10.3 ( ) in Marseille, 10.8 ( ) in Metz, 7.6 ( ) in Rennes, 10.1 ( ) in Toulouse. Conclusions: Although the confidence intervals are wide, the method provides valuable information on the extent of problem drug use. Keywords: capture recapture, problem drug use, multicentric, France.... Introduction he general population surveys do not provide reliable Testimates of problem drug users, which are essential for drug use prevention and treatment policies. 1 Indirect estimation methods, especially capture recapture method, are preferred. 2 The capture recapture technique, originally developed to estimate the size of animal populations, has been adapted by epidemiologists, 3 with two or more data sources. 4,5 The application of the method requires that the captured (i.e. observed) population is an unbiased sample of the whole target population: therefore, data sources should belong to the widest range of activities related to the problem drug use, from the health, social, to criminal aspects. 6,7 Very few studies using this method have been carried out in France: the first one took place in the city of Toulouse in 1995, 8 whereas the last date from the late 1990s. 9 Within the framework of a partnership developed by a European Union agency, the European Monitoring Centre for Drugs and Drug Addiction, France should be in a position to provide results concerning estimated problem drug use. An operational definition could consider that an individual is a problem drug user when his consumption of a substance leads to health, social and/or legal problems for himself or others. Numerous towns across Europe and worldwide possess local estimates of the prevalence of problem drug use French Monitoring Centre for Drugs and Drug Addiction (OFDT), Saint-Denis La Plaine, France. 2 National Institute for Health and Medical Research (Inserm), Paris XI University, Paris, France. Correspondence: Stéphane Legleye, OFDT, 3 avenue du stade de France, Saint-Denis la Plaine, France, tel: , fax: , stleg@ofdt.fr In 2006, the OFDT (French Monitoring Centre for Drugs and Drug Addiction) carried out a capture recapture study in six French urban areas, in which the feasibility of the study was guaranteed: Lille, Lyon, Marseille, Metz, Rennes and Toulouse. Methods Capture recapture We carried out six standard three-sample capture recapture analyses in order to estimate the number of problem drug users in Lille, Lyon, Marseille, Metz, Rennes and Toulouse. Methods have been described in detail elsewhere Participating cities The sample of cities is a convenient one. Choice was made according to the presence of a local network of agencies working in the field of drug use monitoring. Paris was not being included because of its size and the number of social and health agencies working there. In each city, the geographical area was defined taking into account the accuracy and relevance of the already implemented data sources and policies: the urban community of Lille (north of France), with almost 1.1 million inhabitants; Great Lyon (centre south-west) comprising 57 districts with more than 1.15 million inhabitants; inner-marseille (Mediterranean seaside) with almost inhabitants (the districts outside Marseille were not included because very few social and health organizations are to be found there and because their inhabitants rarely visit the specialized centres for drug users in inner-marseille); the Metz (east of France) area covering 61 districts with just over habitants; the Rennes (west) area comprising 10 districts, with almost inhabitants

2 Capture recapture drug users in France 33 Table 1 Combination of data sources in three samples and numbers of records Sample A Sample B Sample C Number of records (%) Number of records (%) Number of records (%) Lille DT, H, GP 1212 (67) LT 327 (18) CO, DS, TP 276 (15) Lyon DT 645 (47) GP 281 (21) CO, LT, DS, TC, TP 437 (32) Marseille DT 470 (61) LT, H 269 (29) CO 95 (10) Metz DT 140 (28) H 156 (31) CO, DS, TP, LT, TC 206 (41) Rennes DT 161 (39) LT 130 (32) CO, TP, GP 118 (29) Toulouse DT, LT 647 (56) GP 424 (37) CO 80 (7) Note: Specialised Drug addiction treatment centres (DT), Low-Threshold services (LT), General Practitioners (GP), Hospital units (H) such as infectious diseases, accident and emergency wards, etc. Drug Squads (DS), users Referred Treatment by the court (RT), Treatment units in Prison (TP), or data detained by the Central Office for the repression of drug-related offences in drug misuse (CO) and the Toulouse (south-east) urban area with 72 districts and inhabitants. Data sources Data were collected over a 6-month period between 2005 and 2006: from these sources: drug treatment agencies, general practitioners, hospital units (such as infectious diseases, attendees at accident and emergency), low threshold agencies, social services and law enforcement sources, such as drug squads, users referred treatment by the court, treatment units in prisons (data collection in prison was delayed for 2 months compared with other data sources in order to allow problem drug users entering prison to have already been captured by other data sources over the last 2 months), and data detained by the OCRTIS (the Central Office for the Repression of drugrelated Offences in drug misuse). Various data sources were combined in three samples following both statistical criteria (odds ratio between two data sources higher than one, suggesting a positive dependency, leads to a combination of both data sources) and a field criterion (when it is locally known that two data sources are positively related). The combination of the data sources in three samples as well as the number of records collected from each sample is shown in table 1. Furthermore, Supplementary table 1 gives details on drugs detected in different cities and figures on polydrug uses. Inclusion criteria Subjects were included in the study if they were resident for >3 months within one of the six cities, if they mentioned illicit drug use over the last 30 days (cannabis excluded): opiates, cocaine/crack, other stimulants and/or hallucinogens (LSD, Ketamine, etc.), and if they were aged years. Individual identifiers The individual identifiers used for matching people from each sample were the initial of the first name, the three first characters of the surname, the sex and date of birth. Due to the nature of the study, no consent of participant was collected. Nevertheless, the participation did not lead to any consequence for the individual, nor medical, social, neither legal. Log-linear models Matched data were analysed using log-linear models. All possible models were run from independent to all two-way interactions (we assumed that there were no three-way interactions). To select the best model, the absolute goodness of fit (G 2, the deviance between observed and expected values) of the model was approximated by the chi-squared distribution. A lower deviance implied that the observed and expected values were closer, indicating that the model fitted better. Models with different degrees of freedom were compared using a log-likelihood ratio test. For models with the same number of interactions, the Akaike Information Criterion (AIC) was used. 28 In each retained model, the expected number for the unknown cell and the associated confidence interval were obtained using the goodness-of-fit-based method. 29,30 Log-linear modelling was computed using the SAS V9.1 software package and the cell counts obtained by the Bishop formula 4 using Microsoft Excel. Ethical approval The study protocol was approved by the CNIL (French National Commission for Data protection and liberties), which ensures that participants freedom and rights were protected. Results Records and overlaps We obtained 1815 records corresponding to 1683 problem drug users in Lille; 7.5% of them were observed in several samples. In Lyon, 1363 records corresponded to 1267 subjects, 7.3% of them were in more than one sample. From 929 records in Marseille, 799 individuals were identified and only 4.2% in several samples. In Metz, 467 individuals were identified from 502 records and 6.8% were present in several samples. In Rennes, 409 records corresponded to 351 persons; 15.4% of them were observed in several samples. From 1151 records in Toulouse, 1090 persons were reported and 5.5% of them were observed in several samples. Overlaps among samples are detailed in Supplementary table 2. Demographic characteristics More than 75% of the subjects were male and half were over 33 years old (table 2). Women were more present in Marseille, Rennes and Toulouse. Mean age ranges from 29.7 in Rennes and 35.2 in Marseille. Most of the individuals lived in stable accommodation: from 59% in Rennes to 74% in Lyon. Nevertheless, unstable accommodation is quite common (30% in Marseille), even being homeless (10% in Rennes). Selected models Table 3 presents the best-fitting log-linear models showing interactions, AIC, G 2 and P-value and their estimates. Models selected in Lille and Rennes contained one significant interaction term between treatment data and harm reduction data. The positive interaction terms (0.16 in Lille and 0.36 in Rennes) suggested that individuals attending the treatment agencies are more likely to also frequent low threshold services. These results seem consistent because social and health services

3 34 European Journal of Public Health Table 2 Age/sex distribution of individuals in each of the six data sets Number in data set Female (%) Age Living status Mean (SD) Stable accommodation (%) Unstable accommodation (%) Homeless (%) Prison (%) Lille (6.48) Lyon (7.75) Marseille (7.37) Metz (7.66) Rennes (7.71) Toulouse (7.85) Table 3 Best-fitting log-linear models: interactions, DF, G2 and AIC, estimates and prevalence Interactions DF G 2 AIC Population aged No. of obs Estimate of unobs Overall 95% CI a Prevalence (ø) 95% CI estimate a Lille Positive: A B Lyon Positive: A C Marseille Independent Metz Independent Rennes Positive: A B Toulouse Negative: A C a: Rounded to nearest 100 work in close collaboration in Lille and Rennes, as reported by local experts. Law enforcement data was independent from other samples. Both models fitted the data well, as demonstrated by the relatively small deviance values. In Metz and Marseille, best models were the independent models. This was consistent with the independent work of the different agencies in these cities. In Lyon and Toulouse, the best-fitting model supposed the statistical independency of the GPs data. This revealed a local trait: general practitioners who met some drug users seem to have very few relationships with other health, social or police services, as reported during the study. The positive interaction in Lyon between social, health and law enforcement workers was quite plausible because some drug users observed by repressive sources are also treated by hospital units. In Toulouse, a negative interaction between police data and treatment data is supported by local perception. Estimates The estimates (table 3) ranged from 1500 problem drug users in Rennes to 8400 in Lyon and 15- to 64-year-old population prevalence ranging from 7.6ø in Rennes to 10.8ø in Lille and Metz. Discussion To estimate the size of the population of problem drug users in six cities, capture recapture analyses were performed using three samples of individuals. We found very similar and credible estimates of the prevalence of problem drug use in the 15- to 64-year-old population in Lille, 10.8ø (range ), in Lyon with 10.6ø (range ), in Marseille, 10.3ø (range ), in Metz, 10.8ø (range ) and in Toulouse, 10.1ø (range ). Nevertheless, the upper limit of confidence interval was somewhat lower in Toulouse and this was corroborated by an examination of other regional indicators which suggested that the prevalence of problem drug use in Toulouse could be slightly less than that in Lyon, Lille, Marseille and Metz. In Rennes, the prevalence appeared lower with a prevalence of 7.6ø (range ). According to local experts, six estimates generated by our models are credible. Comparison with old estimates These last capture recapture survey was conducted in France in 1999 in five cities (Lens, Lille, Marseille Nice and Toulouse) 9 for problem opiates and cocaine users. By restricting our present analysis to the same target group, it is possible to compare our results with those obtained in 1999 for Lille, Marseille and Toulouse. Thus, in 2006 we found a prevalence of problem opiate/cocaine use among the whole population of 6.4ø ( ) in Lille, 6.1ø ( ) in Marseille and 6.7ø ( ) in Toulouse. The prevalence of problem opiate/cocaine users was estimated as follows in 1999: 6.2ø ( ) in Lille, 6.4ø ( ) in Marseille and 4.3ø ( ) in Toulouse. Because the confidence intervals do not overlap each other, the prevalence of problem opiate/ cocaine users in Toulouse seems to be higher in The evolution of the prevalence of the problem opiate/cocaine use between 1999 and 2006 are not significant in Marseille and Lille. These comparisons should be interpreted cautiously since the participating sources and geographical areas are slightly different between 1999 and 2006, due to possible changes in the social, health and/or legal framework of the cities. Comparison with other data A survey among the general 15- to 64-year-old population allows the departmental prevalence of people who experimented with heroine, cocaine, crack, amphetamines, LSD or ecstasy over the previous year to be extracted. 31 Direct prevalence estimates of drug experimentation in six French departments (administrative divisions) are: 16.1ø in Bouche-du-Rhoˆne (Marseille department), 14.1ø in Rhoˆne (Lyon department), 9.3ø in Nord (Lille department), 7.7ø in Haute-Garonne (Toulouse department), 4.9ø in Ille-et-Vilaine (Rennes department) and 0.0ø in Moselle (Metz department). The ranking of the six departments according to these prevalence differed from the ranking of the six cities according to problem drug use

4 Capture recapture drug users in France 35 prevalence. It was not surprising to note that the prevalence of problem drug use in Marseille and Lyon was lower than the drug experimentation prevalence in the general population of Bouche-du-Rhoˆne and Rhoˆne. In Lille, Metz, Rennes and Toulouse, the prevalence of problem drug use is slightly higher than the drug experimentation prevalence in the respective departments. This could be due to a possible and expected general under-registering of drug experimentation in the general population survey as well as to its general methodology: the use of telephone excludes the homeless and the poorest people, and maybe a great part of the problem drug users, and the sampling design does not ensure that data is representative at the department level. When compared with departmental health and social drugrelated indicators in 2004 (available at: ofdtdev/live/donneesloc/indic.html), such as the rate of drug users seeking treatment, injecting material sales, substitution substance sales and the individuals arrested by the police for drug use, our prevalence estimates with capture recapture are consistent. The department of Rennes had the lowest drugrelated indicators in 2004 and was ranked at the bottom of the French departments. On the contrary, the department of Metz is at the top of the French department list in terms of these four drug-related indicators. This could corroborate the relatively high prevalence we found in Metz with capture recapture. The other prevalence of problem drug use in Lille, Lyon, Marseille and Toulouse estimated with capture recapture was relatively closed. The prevalence in Marseille could be under-estimated because a number of data sources declined to participate in the study referring to organizational difficulties in collecting data. Consider the definition of problem drug use The definition of problem drug use per se is a major concern. The substances causing problems, patterns of use, types of problem triggered by drug use and substance legal status may also vary across areas and over time. This is yet another reason for the cautions interpretation of comparisons. The definition of the target group we used is an operational one: in this context, problem drug use refers to all different forms of physical, psychological, social and legal problems caused by the use of opiates, cocaine, stimulants and hallucinogens. As a consequence, individuals who had never encountered any health or social problems due to their drug use but who were in contact with the police on at least one occasion because of their addiction were therefore considered as problem drug users and included in our study. The European definition of problem drug use is as follows: Injecting drug use or long duration/regular use of opiates, cocaine and/or amphetamines. Because France should meet the objectives of the EMCDDA, recent drug users of opiates, stimulants (cocaine, crack, amphetamines) were included in our study, as recommended by the EMCDDA. 32 We also added hallucinogen (LSD, ketamine... but cannabis excluded) users because some fieldworkers at the local level (in particular in Rennes) mentioned that using ketamine is a relevant problem that should be considered as a problem drug use. Nevertheless drug users who consumed only hallucinogens are a very small group and were they excluded of the samples, estimates of problem drug use using capture recapture would not have been very different. Limitations The large confidence intervals we obtained in six studies were probably due to small overlaps between samples. This means that our local estimates should be considered as rough estimates. We took care of the main four underlying assumptions when applying the method The closure assumption means that the true population is unaffected by migration, births and deaths during the study period. Data were examined over a 6-month period in each city and one of the selection criteria had to be present for more than 3 months there: it is therefore to be hoped that the population size was not affected too much by migrations. Moreover, the time span of 6 months is relatively short in drug user s career so that the closed population assumption involving new users (births) entering to the population and others leaving it (i.e. ceasing use, death) can be considered as covered by the data. The second assumption demands that identification of the overlap between different sources is not subject to error. In our protocol, the concise recording of individual identifiers and overlap identification was carefully carried out in order to minimize errors. The heterogeneity assumption may perhaps be the most problematic assumption when applying capture recapture methods. Drug users are diverse and may not have the same probability of being captured depending on gender, on age or on type of substance used. This variability could be controlled by stratifying the population by gender, age group and into three types of substance used (opiates, stimulants, hallucinogens). Nevertheless, the amount of data would be too poor in order to apply capture recapture, particularly for stimulant users and hallucinogen users. Moreover, a significant percentage of poly dug users are present. The Supplementary table 3 shows the distribution of drug detected by different sources. Figures underline that heterogeneity probably exists in Marseille and Toulouse where opiates users are more likely to enter treatment centres than being in the data from the Central office for the repression of drug-related offences in drug misuse. The contrary occurs for stimulant users who are in both cities more likely to be detected by the law enforcement services. In other cities (Lille, Lyon, Metz and Rennes), heterogeneity caused by the type of substance used would not seem to be a major problem. Furthermore, previous studies showed that this does not greatly affect the results 7,10,11,26 and the bias introduced by heterogeneity would appear generally less than that generated by dependencies between samples. 36 The independence assumption, i.e. the absence of a threeway interaction between the three samples in each city, seems reasonable when considering both the chosen models with at least one sample likely to be independent of all the others, and the local situation on the field. The treatment units in prisons collected only few questionnaires, as only a small number of inpatients were included in the sample. Then, the possible bias may only very small if any and we did not detect any tendency to a negative interaction due to the 4 months of overlap with other data sources. Recommendations These approximate local estimates in Lille, Lyon, Marseille, Metz, Rennes and Toulouse seem to be valid, credible estimates. They carry clear implications for professionals and decision-makers at local level and will provide an opportunity to produce a new national estimate of problem drug use in the future. This survey should be repeated in order to produce a follow-up of the size of the problem drug users population. Specific attention should be focussed on the quality of the data-collection stage, especially the professionals involvement in the exhaustive data collection.

5 36 European Journal of Public Health Acknowledgements The authors thank all the participants who collected the data. Supplementary Data Supplementary Data are available at Eurpub online. Funding European and French monitoring centre for drug and drug addiction. Conflicts of interest: The authors are state employees and do declare that they have no financial interest in the funding nor the results. Key points Despite the clear advantages of the capture recapture method compared with general population surveys for problem drug use measurement, no survey had been conducted in France since A multicentric capture recapture survey was conducted in 2006 in six big French cities located in various geographical areas in order to provide French drug use prevalence indicators for France for the European monitoring centre for drug and drug addiction. 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