title: Propensity score estimation to address calendar time-specific channeling in comparative effectiveness research of second generation antipsychotics.
abstract: BACKGROUND
  Channeling occurs when a medication and its potential comparators are selectively prescribed based on differences in underlying patient characteristics. Drug safety advisories can provide new information regarding the relative safety or effectiveness of a drug product which might increase selective prescribing. In particular, when reported adverse effects vary among drugs within a therapeutic class, clinicians may channel patients toward or away from a drug based on the patient's underlying risk for an adverse outcome. If channeling is not identified and appropriately managed it might lead to confounding in observational comparative effectiveness studies.
  
  
  OBJECTIVE
  To demonstrate channeling among new users of second generation antipsychotics following a Food and Drug Administration safety advisory and to evaluate the impact of channeling on cardiovascular risk estimates over time.
  
  
  METHODS
  Florida Medicaid data from 2001-2006.
  
  
  METHODS
  Retrospective cohort of adults initiating second generation antipsychotics. We used propensity scores to match olanzapine initiators with other second generation antipsychotic initiators. To evaluate channeling away from olanzapine following an FDA safety advisory, we estimated calendar time-specific propensity scores. We compare the performance of these calendar time-specific propensity scores with conventionally-estimated propensity scores on estimates of cardiovascular risk.
  
  
  RESULTS
  Increased channeling away from olanzapine was evident for some, but not all, cardiovascular risk factors and corresponded with the timing of the FDA advisory. Covariate balance was optimized within period and across all periods when using the calendar time-specific propensity score. Hazard ratio estimates for cardiovascular outcomes did not differ across models (Conventional PS: 0.97, 95%CI: 0.81-3.18 versus calendar time-specific PS: 0.93, 95%CI: 0.77-3.04).
  
  
  CONCLUSIONS
  Changes in channeling over time was evident for several covariates but had limited impact on cardiovascular risk estimates, possibly due to unmeasured confounding. Although calendar time-specific propensity scores appear to improve covariate balance, the impact on comparative effectiveness results is limited in this setting.
authors.affiliation: Division of General Medicine and Clinical Epidemiology, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America. Dusetzina@unc.edu
authors.forename: Stacie B
authors.initials: SB
authors.lastname: Dusetzina
authors.forename: Christina D
authors.initials: CD
authors.lastname: Mack
authors.forename: Til
authors.initials: T
authors.lastname: Stürmer
mesh_terms: D000293:Adolescent; D000328:Adult; D014150:Antipsychotic Agents / Q000009:adverse effects / Q000494:pharmacology*; D001569:Benzodiazepines / Q000494:pharmacology; D015331:Cohort Studies; D057186:Comparative Effectiveness Research; D003324:Coronary Artery Disease / Q000139:chemically induced; D006801:Humans; D008297:Male; D008875:Middle Aged; D000077152:Olanzapine; D057216:Propensity Score; D016016:Proportional Hazards Models; D013997:Time Factors; D055815:Young Adult
chemical_list: D014150:Antipsychotic Agents; D001569:Benzodiazepines; D000077152:Olanzapine