Identifying heterogeneity among injection drug users: a cluster analysis approach

Objectives. We used cluster analysis to subdivide a population of injection drug users and identify previously unknown behavioral heterogeneity within that population.Methods. We applied cluster analysis techniques to data collected in a cross-sectional survey of injection drug users in Winnipeg, Manitoba. The clustering variables we used were based on receptive syringe sharing, ethnicity, and types of drugs injected.Results. Seven clusters were identified for both male and female injection drug users. Some relationships previously revealed in our study setting, such as the known relationship between Talwin (pentazocine) and Ritalin (methylphenidate) use, injection in hotels, and hepatitis C virus prevalence, were confirmed through our cluster analysis approach. Also, relationships between drug use and infection risk not previously observed in our study setting were identified, an example being a cluster of female crystal methamphetamine users who exhibited high-risk behaviors but an absence or low prevalence of blood-borne pathogens.Conclusions. Cluster analysis was useful in both confirming relationships previously identified and identifying new ones relevant to public health research and interventions.Syringe sharing is a well-established mechanism for the spread of HIV and HCV.1^–3 The introduction of syringe exchange programs (SEPs) has had a substantial impact with respect to decreasing the attributable risk of infectious blood-borne pathogens such as HIV and HCV among injection drug users.4^–11 However, studies have shown variations in the effectiveness of SEPs,11^–18 in that syringe sharing behavior persists among some injection drug users.12^,18^–21Epidemiological studies in which contextual factors are used to explain syringe sharing (i.e., social network analyses22) have demonstrated that sharing behavior is not based exclusively on individual choice7^,23^–26; that is, factors other than syringe access may drive sharing among certain injection drug users.20^,26^–28 For example, syringe sharing has been demonstrated to be associated with dyadic relationships involving close friends or sexual partners12^,26^,29^,30 and partnerships in which injection drug users pool resources to obtain drugs or injection equipment.21^,24^,26^,31^,32 Thus, variation in sharing behaviors is explained by differences between high-risk groups.Some researchers suggest that designing interventions around variances seen between groups, and the context in which these variances reside, may be both efficient and efficacious,33^–38 especially in terms of public health practice. Understanding the heterogeneity in high-risk groups may be particularly relevant for those behaviors that persist despite the establishment of structural interventions such as SEPs.15^,18^–20^,28^,39^–44 Consequently, the use of nontraditional statistical methodologies (such as social network analysis) has been encouraged to capture this variance.33^,35 Cluster analysis,45^,46 “an exploratory technique that can be used to reveal unknown heterogeneity,”35^(p196) focuses on the inherent differences between cases rather than variables. It has been used in the HIV literature to develop typologies of behavior33^,37; however, its use is more widespread in the psychiatric and psychological literatures.38^,47^–50We used cluster analysis as an exploratory tool to investigate whether empirically derived clusters could help to explain heterogeneity in a sample of injection drug users. The broader applicability of this technique to public health investigations in general was also addressed.