Send Orders for_Repjiilts}o- ieji,rints7ii5Jient/iUmscience.nei 130 Currellt Drug Abuse Reviews, 2013,6, 130-143 Methods for Estimating Incidence of Drug Use: A Review Albert Sánchez-Niubo··I, Alessia Mammone2 and Gianpaolo Scalia Tomba2 I Drllg Abuse Epidemiology Research Grollp - IMIM, Barcelona, Spain 2Cenlro Inlerdiparlimenlale di Bioslalislica e Bioinformalica, Universila di Roma Tor Vergala, Rome, ltaly Abstraet: Several methods for estimating drug use incidence, that have been used in the Jiterature or that could be used, having been used in a different framework, are described and commented. The applicability of the different methods depends on available data and knowledge about relevant parameters. The many similarities between drug use incidence estimation and estimation of disease incidence are highlighted, but also the distinguishing aspects that make drug use incidence estimation a challenge to standard statistical methods. Keywords: Drug use, incidence, prevalence, problem drug use, statistical modelling. 1. INTRODUCTlON The incidence of first drug use is an important epidemiological concept that helps understanding the diffusion of drug use in space and time. Incidence estimates can be used to evaluate current and future needs for, and effects of, services and interventions. Incidence is defined as the number of new cases occurring within a given time period, usually ayear. Compared to prevalence, which is defined as the number of existing cases at a certain point in time (or, for practical reasons, in a given period; in studies using administrative data this is often a calendar year), incidence gives more direct infonnation on the recruitment of new cases. It is therefore a more sensitive indicator of effects of prevention efforts than prevalence. However, incidence is directly related to prevalence: adding up the incidence over a number of years, and subtracting the number of cases that have died or ceased to be cases for other reasons, gives the change in prevalence over the same time periodo Note that the incidence of first drug use usually refers to the first use ever. People who relapse after abstinence are not included. Sometimes, the initial event of drug use is called "onset of drug use", but there may be so me uncertainty about the exact detinition of this term, in particular whether this should refer to the "first use ever" or "first habitual use". In this report we will outline some methods that are currently applied or that could be applied for drug use incidence estimation. In the description of the methods, it will often be seen that they were developed with heroin use in mind or for diseases that have the same character as heroin use, namely the progression start of use/infection - continued use/infected state - treatment or other type of registration/diagnosed disease - cessation/cure or death. The typical application in disease epidemiology has be en the estimation of the size of the HIV epidemic assuming AIDS as an observable endpoint. This focus on heroin use or •Addresscorrespondence lo Ihisaulhoral Ihe IMIM-InslitulHospilaldel Mard'InvesligacionsMediques,DoclorAiguader88.E-08003Barcelona. Spain;Tel:+34933160753; Fax:+34933160797; E-mail:asanchez@imim.es 1874-4745/13558.00+.00 equivalently the use of one dominating, strongly addictive drug whose use almost invariably leads to request for treatment or other notifiable events does not always translate well to today's drug scene, where intennittent multi-drug use or use of drugs that rarely lead to statistically observable events has become more and more common. Thus, some of the methods, that have been of common use in the past, now secm dated or limited in scope. However, other methods, such as the model based ones or the survey based ones, that have not been much used in the past, may now become more popular. The described methods are: General inclusion function Retrospective estimation from periodic population surveys Lag correction method Log-linear model of quasi-independence Back-calculation Multistate model with immigration Parametric models Incidence from prevalence Snapshot samples These methods are not altematives to each other. They each have a certain range of applicability, depending on available data. In the following pages the aim is to give some basic understanding of how these methods work and what their requirements, limitations and possibilities are. 2. GENERAL INCLUSION FUNCTlON The tenn "general inc1usion function" (GIF) was probably used for the first time in a paper by Nordt el al. [1]. In that paper, the authors show, using data from the case register for substitution treatment in the canton of Zurich, Switzerland, [rom the period 1992-2004, that the probability of tinding a patient in therapy at a given time depends essentially on the time elapsed since onset of heroin use and not on when this onset occurred. The function rclating these © 2013BenthamSciencePublishers