Estimating the size of people who inject drugs: Utility, means and challenges

Abstract

There are several reasons behind estimating the size of key population e.g. PWID but we broadly can categorize them into two major areas: (i) policy related reasons; and (ii) programme related reasons. The policy related reasons include advocacy, response planning and resource allocation, plus estimations of numbers infected with HIV and projections of the burden of disease. The area of programme related reasons encompass intervention planning, measurement of coverage and monitoring and evaluation of interventions². The estimated size of key population groups including PWID have also been used to garner political support and commitment for the national response for HIV; direct funding by characterizing the extent and pattern of the epidemic; plan HIV programmes for key populations at the national, provincial/state as well as district/city levels; and monitor and evaluate programmes in terms of coverage, quality and effectiveness. The results of a size estimation exercise study can mobilize necessary media attention to demystify so-called age-old traditional notions that the actual number of some key population groups, e.g. PWID, is considerably insignificant and hence not worthy of public health action. Most importantly, size estimates of PWID can be used as denominators for reporting on several international monitoring indicators and for grant applications^1,2.

Today, we have several methods in our knowledge to estimate the size of PWID. However, it is a real fact that each of these available methods has its own strengths. Therefore, it is strongly recommended in various methodological guidelines that we must use multiple size estimation methods, wherever possible, to arrive at a consensus estimate^3-5. All the available size estimation methods can broadly be categorized into two broad categories: (a) direct methods (census and enumeration) and (b) indirect methods (capture–recapture, multiplier, reverse tracking, network scale-up and successive sampling). Direct methods count members of the population directly, whereas indirect methods use data from different sources to estimate their size. In addition, these size estimation methods can also be classified according to their data source: (a) data collected from key populations and (b) data collected from the general population^6-8. For a detailed description of each of these methods, please refer to the recent WHO guidelines document on size estimation and other relevant references.