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The Experience of the Treatment Demand Indicator in Europe: A Common Monitoring Tool Across 30 Countries

Objective: The article describes an epidemiological indicator called Treatment Demand Indicator (TDI). The TDI aims to provide professionals and researchers with a common European methodology for collecting and reporting core data on drug users in contact with treatment services. The article discusses the implementation of the TDI in the European countries and describes the main results, limitations, and future perspectives. Method: The TDI provides a common format for reporting data on clients entering treatment as a result of their drug use and related problems during each calendar year. Its technical protocol defines which clients should be reported at European level and represents the minimum common set of items each national monitoring system should be able to report to the European Monitoring Centre for Drugs and Drug Addiction. Results: In 2015, 29 European countries reported data on 467,811 clients entering drug treatment from 6,846 drug treatment units. Most clients were men in their 30s and had problems related to heroin or cannabis use; patterns of drug use differed geographically. Over the past decade, clients’ profiles and drug use patterns changed from young heroin injectors seeking treatment to drug clients with diversified drug use patterns and profiles. Conclusions: The TDI is the largest drug dataset in Europe, and its data is increasingly used in European and national data analysis. The use of a common drug-treatment-monitoring tool across a group of countries provides a useful instrument for policymakers, professionals, and managers working in the drug treatment field. Objectif : Cet article décrit l’indicateur épidémiologique appelé « Indicateur de la demande de traitement (TDI) ». L’TDI a pour but de fournir aux professionnels et aux chercheurs une méthodologie commune à l’ensemble des pays européens pour collecter et rapporter les principales données à propos des consommateurs de drogues fréquentant les traitements. Cet article traite de l’implantation du TDI dans les pays européens et décrit les principaux résultats, les limites et ainsi que les perceptives futures. Méthode : L’TDI propose un format commun de transmission des données pour chaque année civile à propos des clients qui entrent en traitement pour usage de substances et les problèmes qui y sont associés. Son protocole spécifique détermine quels clients devraient être signalés au niveau européen et décrit les points communs minimaux que chaque système de suivi national devrait être en mesure de transmettre à l’Observatoire européen des drogues et des toxicomanies (OEDT). Résultats : En 2015, 29 pays européens rapportaient des données à propos de 467 811 clients entrant en traitement pour leur consommation de drogues dans 6 846 centres de traitement en toxicomanie. La plupart des clients sont des hommes dans la trentaine et vivent des problèmes liés à leur consommation d’héroïne ou de cannabis; les habitudes de consommation varient selon la géographie. Durant la dernière décennie, le profil des clients et leurs habitudes de consommation ont changé, allant des jeunes injecteurs d’héroïne à la recherche de traitement à des personnes présentant des profils diversifiés d’utilisation de substance. Conclusion : L’TDI est le plus large ensemble de données en Europe et celui-ci est de plus en plus utilisé dans les analyses de données européennes et nationales. L’utilisation d’un outil de suivi commun des traitements en toxicomanie par un ensemble de pays offre un instrument utile pour les décideurs, les professionnels et les gestionnaires qui travaillent dans le domaine des traitements en toxicomanie. Objetivo: El artículo describe el indicador epidemiológico denominado ‘Indicador demanda de tratamiento’. El TDI tiene como objetivo proporcionar a los profesionales y los investigadores una metodología europea común para la recogida y presentación de datos clave sobre los usuarios de drogas en contacto con los servicios de tratamiento. El documento analiza la implementación del TDI en los países europeos y describe los principales resultados, limitaciones y perspectivas futuras. Método: El TDI proporciona un formato común para informar los datos de los clientes que ingresan al tratamiento debido a su consumo de drogas y los problemas relacionados durante cada año (desde 1 Enero hasta 31 Diciembre) . Su protocolo técnico define cuales clientes deben notificarse a nivel europeo y representa el conjunto común mínimo de elementos que cada sistema nacional de supervisión debe poder informar al Observatorio Europeo de Drogas y Toxicomanías (OEDT). Resultados: En 2015, 29 países europeos enviaron datos sobre 467 811 clientes que ingresaron al tratamiento por uso de drogas en 6846 unidades de tratamiento. La mayoría de los clientes eran hombres de entre 30 y tenian problemas relacionados con consumo de heroína o cannabis; los patrones de consumo de drogas difieren geograficamente entre los poises Europeus. En la última década, los perfiles de los clientes y los patrones de consumo de drogas han cambiado desde los jóvenes que se inyectan heroína que buscan tratamiento hasta los consumidores de drogas con patrones y perfiles de consumo de drogas diversificados. Conclusiones: El TDI es la mayor base de datos en el campo de las drogas en Europa y sus datos se utilizan cada vez más en análisis europeas y nacionales. El uso de una herramienta común de monitoreo de tratamiento de drogas en un grupo de países proporciona un instrumento útil para los legisladores, profesionales y gerentes que trabajan en el campo del tratamiento de drogas.

A SUBSTANTIAL PA RT OF DRUG USE is hidden from the observation of classical epidemiological methods (e.g., surveys). Information on the number and characteristics of people entering treatment for their drug use provides insight into general trends in problem drug use and offers a perspective on the organization and uptake of treatment.
The Treatment Demand Indicator (TDI), one of the fi ve key epidemiological indicators 1 of the European Monitor-ing Centre for Drugs and Drug Addiction (EMCDDA), 2 has been implemented to obtain comparable and reliable information on the number, characteristics, and substance use patterns of drug users entering drug treatment in the European countries. In many European countries, treatment data are the primary source of information about high-risk drug use at the national level, and the client population captured by the TDI constitutes a substantial sample of all people with drug problems in the community. The analysis of changes in the profile and in the number of drug users reported through the TDI helps to identify trends in problem drug use. In combination with information on coverage of the TDI within the national treatment system (EMCDDA, 2013), the collected data also offer a perspective on availability, accessibility, and organization of treatment services, which can be useful in the evaluation and planning of drug services.
Information collected through the TDI in 30 countries is largely used to inform the yearly European Drug Report (EMCDDA, 2017a), the Statistical Bulletin (EMCDDA, 2017c), and other EMCDDA thematic publications.
The history of the TDI starts more than 20 years ago, when a fi rst harmonized data collection form was defined at the European Union (EU) level. The fi rst core data set was based on the experience of existing national data collection systems in drug treatment in Europe. In 1994, the Pompidou Group at the Council of Europe, 3 the fi rst European intergovernmental body for collaboration on drug issues, defined a common protocol for collecting data on people entering drug treatment at the city level (Hartnoll, 1994;. Initially, the project covered six cities (Amsterdam, Dublin, Hamburg, London, Paris, and Stockholm) and then developed into a pan-European "multicity" project. The "Pompidou Group protocol" was implemented in several western and southern European countries and from 1993 onward also in central and eastern European cities under a cooperation program between the Pompidou Group and the United Nations Office on Drugs and Crime (UNODC). A revision of the Pompidou Group protocol in the late 1990s led to the publication of the EMCDDA/Pompidou Group TDI protocol 2.0 (Simon et al., 2000).
In 1995, the EMCDDA assumed responsibility for collecting treatment demand data in the member states of the EU. The EMCDDA is a decentralized agency of the European Commission, established in 1993and operational since 1995(Council of the European Union, 2006, with the aim to provide objective, reliable, and comparable information on drugs and drug addiction in the countries of the EU. The organization's role is to provide the EU and its member states with a factual overview of European drug problems and a common information framework to support the drugs debate. 2 See later in the text for a description of the EMCDDA. 3 http://www.coe.int/T/dg3/pompidou To achieve its mission, the EMCDDA coordinates and relies on a network of 30 national monitoring centers, the Reitox national focal points. Reitox, "Réseau Européen d'Information sur les Drogues et les Toxicomanies," is the European information network on drugs and drug addiction (Griffiths et al., 2012). This network is made up of formal national drug observatories in each of the 30 countries reporting to the EMCDDA (the 28 EU member states, the candidate country Turkey, and Norway) and provides annually to the EMCDDA the most up-to-date standardized epidemiological and qualitative information on the drug situation, interventions, and policies at the member-state level. The EMCDDA publishes on a wide range of drug-related topics, across epidemiology, interventions, laws, and policies (Griffiths et al., 2012;Mounteney et al., 2016).
In 2001, the EMCDDA Management Board (EMCDDA, 2001) and the Council of the European Union (Council of the European Union, 2001) formally adopted the TDI as an EU-wide standard, together with four other key epidemiological indicators. The formal endorsement of the indicators supported their implementation at the national level.
In 2012, the TDI protocol was revised, with the publication of the TDI Protocol version 3.0 (EMCDDA, 2012). The revised TDI protocol reflected changes in patterns of drug use and client profiles (i.e., diversified patterns and clients), in the drug treatment systems (i.e., expansion and diversifi cation of treatment), and in methodological and technical developments in data monitoring (i.e., development of an information technology system).
The TDI protocol 3.0 is currently available online in 12 languages, 4 and it is used as a data collection tool in drug treatment in the 28 countries of the EU, 5 Norway, and Turkey, as well as in some countries of the Balkan region, the Mediterranean area, North and West Africa, and South America.
This article provides an overview of the TDI and its implementation in the European countries in the last 10 years by describing the latest available data and discussing the methodological aspects, limitations, and future perspectives for the development of the indicator.

Method
The TDI provides a uniform structure for reporting data on clients seeking care in drug treatment facilities at, or close to, their point of entry into treatment for problems 4 http://www.emcdda.europa.eu/publications/manuals/tdi-protocol-3.0 5 On January 1, 2018, the 28 European member states were as follows: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, and the United Kingdom. with one or more drugs. It is designed to satisfy monitoring requirements at the European level. In addition, it is envisaged that each country or member state supplements the common core data set with additional variables to inform local or national data collection needs.
Data are collected on two groups of clients: those entering drug treatment for the fi rst time during the reporting year ("clients entering treatment") and, among them, those entering treatment for the fi rst time in their life ("first-time treatment entrants"). The recommended period for data collection indicated in the protocol is from January 1 to December 31.
To ensure comparability, specific indications are provided for inclusion and exclusion of clients in the TDI register according to their treatment episode (start/end of treatment and treatment episode) (EMCDDA, 2012). For each presenting client, a core data set of 24 items is reported anonymously concerning sociodemographic characteristics of the client, patterns of drug use, and use of services (Table 1).
TDI data are collected in the centers that provide specialized drug treatment, which is defined as "any activity which directly targets people who have problems with their drug use and aims to achieve defined objectives with regard to the alleviation and/or elimination of these problems, provided by experienced or accredited professionals, in the framework of recognized medical, psychological or social assistance practice." Five types of treatment centers can be included in the data collection: (a) outpatient treatment centers, (b) inpatient treatment centers, (c) low-threshold agencies, (d) treatment units in prison, and (e) general practices.
The TDI collects data by the primary drug, which is defi ned as the drug that causes the client the most problems at the start of treatment. This is usually based on the request made by the clients and/or on the diagnosis made by a therapist, commonly using standard clinical instruments (e.g., the International Classification of Diseases,10th Revision;Diagnostic and Statistical Manual of Mental Disorders,Fifth Edition;and Addiction Severity Index). Up to four additional drugs that create problems for the clients can be recorded in addition to the primary drugs, and they are defined as secondary drugs. 6 The broad drug categories included in the TDI protocol are opioids, cocaine, stimulants other than cocaine, cannabis, hypnotics and sedatives, hallucinogens, volatile substances/inhalants, and other substances (not included in the categories above). Each category is broken down into specific substances. According to the EMCDDA regulation (Council of the European Union, 2006), only illicit drugs and licit drugs in the context of polydrug use are subject to EMCDDA monitoring. Therefore, the TDI does not collect data on tobacco, and it collects data on alcohol only if it is used together with an illicit drug. Data are collected at the level of the treatment centers, collated at the national level, and transmitted to the Reitox national focal points (Griffiths et al., 2012), which send aggregated data to the EMCDDA's online database Fonte once per year. This process can result in a 1.5-year delay between the year when data are collected at the local level and the year when the data are reported to the EMCDDA. After submission, data go through a process of quality assurance lasting several months. During this period, data are checked for several parameters (e.g., internal and temporal consistencies, internal and external validities, thorough cross-checking with other data and information sources), and a regular dialogue with experts from the 30 EU countries allows the EMCDDA to ensure data quality.
A number of ethical issues should be considered when TDI data are collected. The main ethical principles described in the TDI protocol concern the purpose of data collection and data protection. Data should be collected with the purpose of obtaining basic information on the epidemiological situation and of supporting evidence-based health interventions. For every level of information (e.g., clinical, regional, national, and institutional), national and international rules of confidentiality and data protection must be considered, as well as the rights of clients, staff, and treatment centers (EMCDDA, 2012).

Data 2015
In 2015 (or the most recent year available before 2015), 29 7 European countries (EU-29) reported data on 467,811 clients entering drug treatment from 6,846 drug treatment units; 37% of these were fi rst-time treatment entrants (N = 172,303). Five large EU countries (France, Germany, Italy, Spain, and the United Kingdom)-where 53% of the European adult population live-reported more than 70% of drug clients entering treatment in Europe (Table 2).
Eighty percent of fi rst-time treatment entrants were male, with a mean age of 33 years. The male-to-female ratios were 2:1 in the Czech Republic, Finland, and Norway; 19:1 in Turkey; and 8:1 in Cyprus. The mean age at treatment entry ranged from 23 years in Croatia to 37 years in Italy. Thirty-seven percent of the clients were unemployed (compared with 9.4% in the general population); 7% were homeless and/or without a stable accommoda-7 Countries included were Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Poland, Portugal, Slovakia, Slovenia, Spain, United Kingdom, Norway, and Turkey. Sweden was not included in data reported because most definitions were not in line with the EMCDDA definitions. Norway is not included in the analysis on fi rst-time treatment entrants because data were not available.  ing treatment in Europe in 2015, the largest group of drug clients entered treatment for problems related to primary use of opioids, mainly heroin (38%), followed by those entering treatment for primary use of cannabis (31%), and those entering treatment for primary use of cocaine (13%), stimulants other than cocaine (mainly amphetamines and methamphetamines) (8%), and other substances (10%), including hypnotics and sedatives, hallucinogens, and volatile substances/inhalants (Table 3). The proportion of clients entering drug treatment for problems related to the primary use of opioids ranged from 4% in Hungary to 93% in Estonia; that of primary cocaine clients varied from 0% in Finland to 37% in Spain; primary cannabis clients ranged from 3% in Bulgaria to 71% in Denmark; and the proportion of clients entering treatment for stimulants other than cocaine ranged from 1% or less New psychoactive substances (NPSs) 9 (Council of the European Union, 2005) are included in the group of "other substances." These are mainly synthetic cannabinoids and cathinones and represent the main reasons for entering treatment for about 3,000 clients in Europe, or less than 1% of clients entering treatment in 2015, and are mostly reported by Poland, the Netherlands, and the United Kingdom.
When looking at fi rst-time treatment entrants, 45% entered treatment for problems related to primary cannabis use; fewer entered for using opioids (20%), cocaine (15%), and 9 NPSs are defined as new narcotic drugs or new psychotropic drugs in pure form or in a preparation that have not been listed under the 1961 United Nations Single Convention on Narcotic Drugs (Schedule I, II, or IV) or under the 1971 United Nations Convention on Psychotropic Substances (Schedule I, II, III, or IV). stimulants other than cocaine (10%). The remaining clients entered treatment for primary use of other substances (Table  4). In Europe, injecting drug use is most commonly associated with opioids, although in a few countries, the injection of stimulants such as amphetamines or cocaine is a problem. Nineteen percent of clients entering treatment in 2015 injected their primary drug (EMCDDA, 2017a).

Trends
To provide a description of treatment incidence and recent trends in drug treatment demand, the analysis of changes over time in the primary drug of use among treatment clients has focused on fi rst-time treatment entrants.
Over the past two decades, data collected from people entering drug treatment have revealed substantial changes in the prevailing patterns of drug use. Sufficient data to de-  (Figure 1) (EMCDDA, 2017a).
Of fi rst-time treatment entrants between 2006 and 2015, the number of opioid clients decreased from 56,000 (37%) to 33,000 (21%). Between 2014 and 2015, 10 countries reported an increase in the number of opioid (mainly heroin) clients, 11 and six countries 12 reported an increase in the proportion of heroin clients out of all fi rst-time treatment entrants. Among opioid clients, those using heroin more than halved from a peak of 59,000 (37% of fi rst-time treatment entrants) in 2007 to 23, 000 (19%) in 2013 before increasing again to 29,000 (21%) in 2015 (EMCDDA, 2017a).
At the same time, in most European countries the number of fi rst-time treatment entrants for primary cannabis use has continuously increased from about 43,000 (28% of all new drug clients) in 2006 to about 75,000 (47% of all new drug clients) in 2015.
Increases have also been reported in the number of fi rst-time treatment entrants for primary amphetamines and methamphetamines use, from approximately 9,000 (6% of all new drug clients) in 2006 to approximately 15,000 (9% of all new drug clients) in 2015, although these were mainly driven by increases in Germany and, to a lesser extent, in the Czech Republic.
Trends in cocaine treatment demands, concentrated in a few countries, show that the overall number of fi rst-time treatment entrants for cocaine declined from a peak of about 35,000 (23%) in 2006 to about 26,000 (17%) in 2015 ( Figure 2).
Drug users consuming other substances have recently appeared in drug treatment data in some European countries, such as users of opioids other than heroin (e.g., methadone or buprenorphine misused) or other types of opioid medi-cines (e.g., tramadol, oxycodone) and some types of NPSs, such as synthetic cannabinoids and cathinones. Between 2006 and 2015, a decline from 28% to 19% of fi rst-time treatment entrants with injection as the main route of administration was reported (EMCDDA, 2017a).

Discussion
According to TDI data, the typical European client entering drug treatment is a man in his 30s, often unemployed, and with unstable living conditions and a low level of education. Among opioid clients, many re-enter drug treatment after one or more previous treatment episodes (70% of opioid clients have been treated before) (EMCDDA, 2017c), and the mean age has increased by 4 years between 2006 and 2015, indicating the presence of an aging cohort of drug users (Johnston et al., 2017). A history of injecting drug use and poor health, unstable living conditions, and tobacco and alcohol use makes these users susceptible to a range of chronic health and social problems that may accelerate their  Cocaine  cocaine  sedatives  nogens  inhalants  Cannabis  substances  Total   Austria  2015  351  125  89  25  3  0  711  3  1,307  Belgium  2015  411  756  392  314  19  3  2,065  41  4,001  Bulgaria  2014  207  21  53  5  0  1  27  7  321  Croatia  2015  176  24  30  41  1  0  526  50  848  Cyprus  2015  50  36  17  1  1  -330  -435  Czech Republic  2014  333  12  3,553  31  6  7  776  10  4,728  Denmark  2014  124  134  151  18  4  4  1,783  39  2,257  Estonia  2015  55  -2  --1  5  63  Finland  2015  106  0  51  16  3  1  99  1  Notes: Cocaine includes powder cocaine, crack cocaine, and "other." Data for Norway are for clients entering specialized treatment and are not fully in line with the Treatment Demand Indicator (TDI) protocol 3.0. Data for Sweden are excluded due to comparability problems with the TDI protocol 3.0 definitions. No data on fi rst-time treatment entrants are available for Norway, so data on clients entering drug treatment were included. Data from Bulgaria, the Czech Republic, Denmark, and Spain refer to 2014 because that was the last year of available data. Tx = treatment.
physical aging and have considerable implications for treatment and social support services (EMCDDA, 2017a). Women entering treatment make up around one fi fth of all entrants to drug treatment in Europe. High male-to-female ratios are reported in the southern European countries and lower ratios in the northern European countries. Women have specific needs that have to be addressed because they are likely to experience stigma and economic disadvantage, have less social support, come from families with substance use problems and have a substance-using partner, have children who may play a central role in their drug use and recovery, have experienced sexual and physical assault and abuse, and have co-occurring mental disorders (EMCDDA, 2017b).
The social profile of clients entering treatment in European countries shows that clients are more socially disadvantaged than the general population, 13 providing indications for prevention and for addressing social risk factors for drug-related problems.
Concerning drug use patterns, the predominance of primary heroin users entering drug treatment at the beginning of this century in Europe is now replaced by a more diversifi ed treatment client population and by a major increase in demands for cannabis treatment in the last years. An analysis of long-term trends in fi rst heroin treatment demand from 2000 until 2009 showed that the trend peaked in 2007 and then significantly decreased until 2009; the decreasing started earlier in the western European countries and later in the countries of the eastern part of Europe (Barrio et al., 2013). Factors that could have played a role in the rise of cannabis treatment demands are increases in the prevalence of can-People_in_the_EU_%E2%80%93_statistics_on_housing_condi tions#Severe_housing_deprivation; educational level: http://ec. europa.eu/eurostat/statistics-explained/index.php/Glossary:Early_ leaver_from_education_and_training. nabis use in some countries, in particular intensive cannabis use (Sánchez-Niubò et al., 2013), treatment availability targeting cannabis use, cannabis market factors (potency and cannabis availability), and changing treatment referral practices (Montanari, 2017).
Drug injection has also substantially decreased in the last 10-15 years. An analysis of TDI data from 2001 until 2011 indicates a decline in injecting drug behavior and its incidence among people admitted to fi rst drug treatment in the 28 EU member states, Norway, and Turkey (Sarasa et al., 2015).
Country differences in patterns of drug use and in client profiles should be considered when describing the European treatment demand data. Clients entering treatment for the use of non-cocaine stimulants are mainly concentrated in the Czech Republic and Slovakia (methamphetamines), and in Bulgaria, Finland, Germany, Hungary, Latvia, and Poland (amphetamines) . The use of co- caine among those demanding treatment is mainly reported in Italy, the Netherlands, Spain, and the United Kingdom (Espelt, 2015); the United Kingdom is the country reporting the most crack cocaine treatment demands in Europe (EMCDDA, 2017c). Furthermore, in Finland, the majority of drug clients seek treatment for problems related to misuse of buprenorphine (Aalto et al., 2007) and, in Estonia, for problematic use of illicit fentanyl (Ojanperä et al., 2008).
Country differences reflect variations in cultural and social factors, extent of drug use and drug use problems in the country, different availability and organization of national treatment programs, drug policies, referral practices, and other possible factors.
Finally, NPSs may pose challenges for users and drug treatment services, but only few persons are reported to enter treatment for the use of NPSs as a primary drug, reflecting low prevalence levels of these substances in the general population, lack of available services, and underreporting.

Limitations
When describing and interpreting data collected through the TDI in 30 countries across Europe, data limitations should be considered (EMCDDA, 2016;Mounteney et al., 2016). First, TDI data only represent a part of the drug treatment system, and its level of representativeness may differ by country. Information on the national treatment system and the coverage of the TDI within it are essential prerequisites to understand the context of TDI data collection in each country and how the data coming through the TDI reporting system mirror the actual overall uptake of treatment services in the country. Currently, the data coverage of specialized outpatient and inpatient treatment facilities in most countries is reasonable, whereas for other treatment providers (e.g., general practitioners or low-threshold agencies) the coverage remains low.
Second, national differences in operational definitions limit data comparability. Data are collected according to the same TDI protocol in all reporting countries, based on the same 24 items; however, some differences between countries and within countries in the way data are collected remain. This is particularly the case in countries with a federal organization that gives large autonomy to individual provinces or regions. To interpret the reported data, it is necessary to have a clear understanding of national and subnational details in data collection.
Third, trends should be interpreted with caution, as fl uctuations in data reporting, including in the number and type of units reporting data and the number of clients reported, may be influential. Specific analysis of heroin and cannabis trends has been conducted to isolate those factors, but ad hoc analysis is required to better understand the reporting bias.
A fourth limitation is the double counting of individuals. According to the TDI protocol, single individuals are recorded, and not episodes; however, it may happen that a person is registered more than once in a database, which results in an overestimation of the total number of persons entering treatment in a given year. Reasons for double counting include lack of communication between treatment centers and absence of a unique system to allow for crosschecking. Although several techniques exist (Origer, 1999) and substantial progress has been made in recent years, double counting of cases can only partly be excluded.
A fi fth limitation of the indicator is the restricted range of substances included in data collection. As the mandate of the EMCDDA is limited to illicit drugs, the TDI does not collect information on tobacco, and it collects information on alcohol only if it is reported as a secondary drug for entering treatment. Other forms of addiction, such as gambling, are also not included in the TDI.
Furthermore, the TDI only collects information on the client at treatment entrance, and no information is available on the other phases of the treatment process, including treatment exit and outcome. An additional module of the TDI (TDI Prevalence), aiming at collecting data on the number and profile of people who are in continuous treatment (not only entering), was implemented in seven countries in 2015, 14 with possible expansion in the future.
The last data quality issue is the way data are transmitted from the national to the European level. Treatment monitoring at the EU level is based on the transfer of aggregated data from national sources, which limits the possibilities of the EMCDDA both in terms of quality control and analysis.
To address these issues and improve data quality, a system of data quality assurance has been established. This includes the process of data validation conducted every year, as well as the regular dialogue with the TDI experts nominated in every country. In addition, a detailed system of data quality assessment is conducted every 3 years to scrutinize the data provided by each country for specific criteria of scientific quality and process of TDI implementation at the national level.

Conclusions
Since its implementation, the TDI has become the largest drug-related data set in Europe, and it is used in various analyses of the drug situation and trends. The use of a common data collection tool across countries represents a powerful instrument for policymakers at the European level and allows drug professionals to compare their national situation in drug treatment and drug problems with other countries in order to improve their national responses. Data collected in 30 countries through the TDI remain the main information source on the extent and patterns of drug problems and sociodemographics of individuals experiencing drug problems and in contact with treatment services in the EU member states, Norway, and Turkey.
The success of the TDI in monitoring the drug situation in Europe over the last 20 years is the fruit of a collaborative effort between the EMCDDA and its network of national experts. Further efforts will be required for the TDI to reflect and adapt to fast-paced changes in the drug markets (e.g., NPSs) and a greater diversification of health care systems (e.g., increased importance of e-health).