Methods and Problems with Estimating Prevalence of People Who Inject Drugs, studies and debate on how to get better and more consistent estimates in Europe and Rest of the World

Knowing the size of the population of people who inject drugs (PWID) living in each country is necessary in order to quantify the burden of disease associated with injecting drug use and to plan harm reduction and health services accordingly. Estimates are usually obtained through indirect statistical methods and come with a high degree of uncertainty. In some cases, there are discrepancies between estimates of the prevalence of PWID and people dependent on opioids. In others, the number of reported PWID has fallen over time. While this might reflect a real trend away from injecting drug use among opioid users in some countries, this might also be influenced by the different estimation methods adopted over time (e.g. switching from standard indirect methods to population surveys or hotspot mapping techniques), which means estimates of the coverage of treatment and harm reduction interventions and denominators required for assessing trends in drug related harm also may be inconsistent or biased. We aim to present some of the methods and challenges encountered in Europe and globally on estimating prevalence of PWID and showcase recent methodological developments and new indirect estimates to inform a debate on what we can recommend to improve prevalence estimation in Europe and elsewhere.