Diagnosis of hepatitis C virus infection in Scotland's injecting drug user population.

We estimated the extent of undiagnosed hepatitis C virus (HCV) infection in injecting drug users (IDUs) in Scotland. We used record-linkage to determine HCV diagnosis status for 41 062 current/former IDUs attending drug treatment and support services between 1 April 1995 and 31 March 2006; the extent of undiagnosed HCV infection was estimated by comparing the number HCV-diagnosed to the number HCV-infected (estimated from an unlinked anonymous testing survey of 2141 current/former IDUs). In all, 9145 IDUs (22%) were diagnosed HCV antibody-positive since first attendance at drug services (diagnosis rate of 33.6/1000 person-years, 95% CI 32.7-34.4). By 31 March 2006, of the 19 632 current/former IDUs who had attended drug services and were determined to be living with HCV, an estimated 58% (95% CI 45-62) had not been HCV-diagnosed. It is essential that the deployment of resources for identifying at-risk IDUs with a view to offering antiviral therapy is guided by evidence.