Introduction
This report presents the development and methodological guidelines for the TDI Treatment Prevalence project, a European initiative launched in 2006 to complement the existing Treatment Demand Indicator (TDI) by capturing data on clients already in drug treatment at the start of the year. Recognising that the standard TDI only accounts for new entries into treatment, the project addresses a significant gap in monitoring long-term and ongoing treatment cases —particularly among opioid users — across EU Member States. The guidelines aim to harmonise data collection practices and provide a more comprehensive understanding of the total treated population, informing estimates of treatment coverage, service capacity, and patterns of drug use within the treatment system.
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