title: Representativeness of an HIV cohort of the sites from which it is recruiting: results from the Ontario HIV Treatment Network (OHTN) cohort study.
abstract: BACKGROUND
  Participation bias is a well-known phenomenon in epidemiologic research, where individuals consenting to research studies differ from individuals who are not able or willing to participate. These dissimilarities may limit the generalizability of results of research studies. Quantification of the participation bias is essential for the interpretation of research findings.
  
  
  METHODS
  The Ontario HIV Treatment Network Cohort Study (OCS) is an ongoing open cohort study of HIV positive individuals receiving care at one of 11 sites in Ontario. OCS participants from 4 sites were compared to non-participants (those who declined or were not approached) at those sites with regard to gender, age, HIV risk factor, CD4 count and viral load (VL). Generalized logit regression models were used to identify predictors of declining to participate or not being approached to participate.
  
  
  RESULTS
  Compared to participants (P) in the OCS, individuals who declined to participate (D) and those who were not approached (NA) were slightly younger (D:45, NA:44 vs P:46), less likely to be male (D: 71%, NA:75% vs P:88%), less likely to be Caucasian (D:41%, NA:57% vs P:72%) and less likely to be Canadian-born (D: 39%, NA: 52% vs P: 69%). Patients who were not approached to participate were less likely to have VL < 50 copies/mL than other patients (D: 75%, NA: 62%, P: 74%) and had lower CD4 counts than OCS participants (D: 450 cells/mm3, NA: 420 cells/mm3, P: 480 cells/mm3).
  
  
  CONCLUSIONS
  Significant demographic and clinical differences were found between OCS participants and non-participants. Extrapolation of research findings to other populations should be undertaken cautiously.
authors.affiliation: Toronto General Research Institute, University Health Network, Room 13EN226, 200 Elizabeth St., Toronto, Ontario M5G 2C4, Canada. jraboud@uhnresearch.ca
authors.forename: Janet
authors.initials: J
authors.lastname: Raboud
authors.forename: DeSheng
authors.initials: D
authors.lastname: Su
authors.forename: Ann N
authors.initials: AN
authors.lastname: Burchell
authors.forename: Sandra
authors.initials: S
authors.lastname: Gardner
authors.forename: Sharon
authors.initials: S
authors.lastname: Walmsley
authors.forename: Ahmed M
authors.initials: AM
authors.lastname: Bayoumi
authors.forename: Sandra
authors.initials: S
authors.lastname: Blitz
authors.forename: Curtis
authors.initials: C
authors.lastname: Cooper
authors.forename: Irving
authors.initials: I
authors.lastname: Salit
authors.forename: Jeff
authors.initials: J
authors.lastname: Cohen
authors.forename: Sean B
authors.initials: SB
authors.lastname: Rourke
authors.forename: Mona R
authors.initials: MR
authors.lastname: Loutfy
mesh_terms: D000328:Adult; D018791:CD4 Lymphocyte Count; D002170:Canada / Q000453:epidemiology; D015331:Cohort Studies; D005260:Female; D015658:HIV Infections / Q000453:epidemiology* / Q000821:virology; D015497:HIV-1 / Q000145:classification; D006801:Humans; D008297:Male; D008875:Middle Aged; D009864:Ontario / Q000453:epidemiology; D010349:Patient Compliance; D015983:Selection Bias; D011795:Surveys and Questionnaires; D019562:Viral Load