Estimating the number of people with hepatitis C virus who have ever injected drugs and have yet to be diagnosed: an evidence synthesis approach for Scotland

Abstract

Aims: To estimate the number of people who have ever injected drugs (defined here as PWID) living in Scotland in 2009 who have been infected with the hepatitis C virus (HCV) and to quantify and characterize the population remaining undiagnosed.

Methods: Information from routine surveillance (n=22616) and survey data (n=2511) was combined using a multiparameter evidence synthesis approach to estimate the size of the PWID population, HCV antibody prevalence and the proportion of HCV antibody prevalent cases who have been diagnosed, in subgroups defined by recency of injecting (in the last year or not), age (15-34 and 35-64 years), gender and region of residence (Greater Glasgow and Clyde and the rest of Scotland).

Results: HCV antibody-prevalence among PWID in Scotland during 2009 was estimated to be 57% [95% CI=52-61%], corresponding to 46657 [95% credible interval (CI)=33812-66803] prevalent cases. Of these, 27434 (95% CI=14636-47564) were undiagnosed, representing 59% [95% CI=43-71%] of prevalent cases. Among the undiagnosed, 83% (95% CI=75-89%) were PWID who had not injected in the last year and 71% (95% CI=58-85%) were aged 35-64 years.

Conclusions: The number of undiagnosed hepatitis C virus-infected cases in Scotland appears to be particularly high among those who have injected drugs more than 1 year ago and are more than 35 years old.

Keywords: Evidence synthesis; hepatitis C; people who inject drugs; prevalence.

Figure 1

Individuals aged 15–64 years diagnosed with hepatitis C virus (HCV) in Scotland by the end of 2009 (and not known to have died) by risk group, as recorded on the Hepatitis C Diagnosis Database (DiagDat) and linked Drugs Misuse Database (TrtDat) data. Recent and non‐recent PWID (people who inject drugs) refers to status in 2009

Figure 2

Capture–recapture (CR) estimates of the number of recent people who inject drugs (PWID) used as prior distribution for T_d ρ_R,d in evidence synthesis model and posterior distributions for size of the recent PWID population as estimated by evidence synthesis models, with and without bias adjustment parameters, by region of residence, gender and age. CR prior distribution, posterior distribution for model with bias adjustment parameters (baseline model), posterior distribution for model without bias adjustment parameters (sensitivity 2)

Figure 3

Relationship between the parameters and the data sources. Circles denote the unknown parameters (or functions of parameters) which are to be estimated. Squares denote the data sources. A link between a parameter (or function of parameters) and a data source indicates that the data source provides information on that parameter (or function of parameters). ρ : proportion of the population in risk group; π : hepatitis C virus (HCV) prevalence; δ : proportion of HCV‐prevalent cases that are diagnosed; T : total population size; b^D : bias parameter for the number of diagnosed people who inject drugs (PWID) recently; b^δ : bias parameter for proportion diagnosed