title: Short look-back periods in pharmacoepidemiologic studies of new users of antibiotics and asthma medications introduce severe misclassification.
abstract: OBJECTIVE
  The aim of this study was to quantify the effect of the look back period on the misclassification of new users of antibiotics and asthma medications.
  
  
  METHODS
  We included all children born in Denmark from 1995 through 2006 and all prescriptions of antibiotics and asthma medication from 1995 through 2011. The study period was 2007 through 2011. True new users redeemed their first prescription in the study period whereas prior users redeemed their first prescription before the study period. Look-back periods ranged from 30 days up to 12 years prior to the study period, and we defined new users as those without a prescription in the look-back period. The relative misclassification (RM) was estimated as the number of defined new users divided by the number of true new users.
  
  
  RESULTS
  For antibiotics, the RM decreased from 4.75 for look-back periods of 30 days to 2.36 for 2 years and 1.33 for 5 years. For asthma medication, the RM decreased from 2.53 for look-back periods of 30 days to 1.48 for 2 years and 1.20 for 5 years. Older age, male gender, and absence of treatment-related diagnoses were associated with higher RM.
  
  
  CONCLUSIONS
  Studies applying the new user design are strongly dependent on the available information on prescriptions. For drug classes with intermittent use such as asthma medications, even a 2-year look-back period produced severe misclassification. Excluding children with a prior treatment-related diagnosis can reduce the level of misclassification.
authors.affiliation: Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus N, Denmark.
authors.forename: Anders H
authors.initials: AH
authors.lastname: Riis
authors.forename: Martin B
authors.initials: MB
authors.lastname: Johansen
authors.forename: Jacob B
authors.initials: JB
authors.lastname: Jacobsen
authors.forename: M Alan
authors.initials: MA
authors.lastname: Brookhart
authors.forename: Til
authors.initials: T
authors.lastname: Stürmer
authors.forename: Henrik
authors.initials: H
authors.lastname: Støvring
mesh_terms: D018927:Anti-Asthmatic Agents / Q000008:administration & dosage*; D000900:Anti-Bacterial Agents / Q000008:administration & dosage*; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D003718:Denmark; D017723:Drug Utilization Review / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D008297:Male; D017891:Pharmacoepidemiology / Q000379:methods* / Q000706:statistics & numerical data; D012189:Retrospective Studies; D013997:Time Factors
chemical_list: D018927:Anti-Asthmatic Agents; D000900:Anti-Bacterial Agents
keywords: antibiotics; asthma medications; methods; misclassification; new users; pharmacoepidemiology