title: Trajectories of injecting behavior in the Amsterdam Cohort Study among drug users.
abstract: BACKGROUND
  Injecting frequency among people who inject drugs (IDU) can change along distinct trajectories, which can reflect on incidence of HIV and HCV infections. We aimed at assessing these patterns of longitudinal changes, their predictors and their association with the incidence of HIV and HCV.
  
  
  METHODS
  We analyzed data from the Amsterdam Cohort Study among Drug Users, selecting participants recruited from 1985 to 2005, injecting drugs before cohort entry and with records in at least three different six months intervals (N=740). We used latent class mixed models to identify distinct trajectories of injecting, multinomial regression to identify socio-demographic variables associated with those patterns and Kaplan-Meier analysis for the estimation of the corresponding cumulative HIV and HCV incidence.
  
  
  RESULTS
  Five distinct patterns for injecting frequency and for injecting since last visit were identified. The majority of participants (three groups, 69% of participants) had stable risk injecting behavior; the remaining displayed a decrease in injecting over time. Those with longer duration of injecting at cohort entry and those who entered the cohort in earlier years tended to have continuing high risk behavior. The HIV risk was highest among those with continuing high risk behavior and its changes over time mirrored the patterns of change in injecting in a group with decrease in injecting.
  
  
  CONCLUSIONS
  Individual longitudinal patterns of changes in injecting behavior are related to socio-demographic and drug use variables and are reflected in the incidence of HIV infections. Understanding these associations might provide valuable information for targeted interventions.
authors.affiliation: Department of Epidemiology, Helmholtz Centre for Infection Research, Braunschweig, Germany; Hannover Medical School, Hannover, Germany. Electronic address: rafael.mikolajczyk@helmholtz-hzi.de.
authors.forename: Rafael T
authors.initials: RT
authors.lastname: Mikolajczyk
authors.affiliation: Hannover Medical School, Hannover, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Horn
authors.affiliation: Department of Internal Medicine, Academic Medical Centre, CINIMA, Amsterdam, The Netherlands; Public Health Service Amsterdam, Cluster Infectious Diseases, Department of Research, Amsterdam, The Netherlands.
authors.forename: Maria
authors.initials: M
authors.lastname: Prins
authors.affiliation: European Monitoring Centre for Drugs and Drug Addiction (EMCDDA), Lisbon, Portugal.
authors.forename: Lucas
authors.initials: L
authors.lastname: Wiessing
authors.affiliation: Julius Centre for Health Sciences and Primary Care, University Medical Centre Utrecht, Utrecht, The Netherlands; Centre for Infectious Disease Control, RIVM, Bilthoven, The Netherlands.
authors.forename: Mirjam
authors.initials: M
authors.lastname: Kretzschmar
mesh_terms: D000328:Adult; D016739:Behavior, Addictive / Q000175:diagnosis / Q000453:epidemiology / Q000523:psychology; D015331:Cohort Studies; D055030:Drug Users / Q000523:psychology*; D005260:Female; D015658:HIV Infections / Q000175:diagnosis / Q000453:epidemiology* / Q000523:psychology*; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D009426:Netherlands / Q000453:epidemiology; D011446:Prospective Studies; D012309:Risk-Taking; D015819:Substance Abuse, Intravenous / Q000175:diagnosis / Q000453:epidemiology* / Q000523:psychology*; D055815:Young Adult
keywords: Behavioral change; Finite mixture models; Injecting drug use