Injecting drug use is the main route of transmission for the hepatitis C virus in the developed world. Knowledge about the characteristics of the past and current injecting drug user (IDU) population is therefore vital in order to understand the epidemiology of hepatitis C. The IDU population is 'hard to reach' and hence most epidemiological studies have concentrated on estimating current IDU prevalence, whilst little is known about the potentially large pool of ex-injectors. We demonstrate a method for estimating the proportion of ex-users in the population, by considering injecting drug use as a time-to-event process. We show how unbiased estimates of injecting duration and historical patterns in injecting initiation can be derived from a sample of ex-IDUs obtained from a population survey, and how such data lead to estimates of the proportion of ex-IDUs in the population. Finally, we show how to obtain estimates of the prevalence of ex-IDUs by using additional information on the prevalence of current IDUs.
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