title: Revisiting the washout period in the incident user study design: why 6-12 months may not be sufficient.
abstract: OBJECTIVE
  The purpose of this study was to describe how washout period duration affects the size and accuracy of retrospective incident user cohorts.
  
  
  METHODS
  MarketScan commercial claims data from 2007 to 2010 were used and included adults with an antihyperlipidemic, antidiabetic or antidepressant claim in 2010. Incident user cohorts using 3-, 6-, 12-, 24- and 36-month washouts were created and changes in sample size and incident user misclassification were described.
  
  
  CONCLUSIONS
  The 6- and 12-month washouts excluded 75 and 85% of the samples, respectively. Half of subjects in the 6-month washout cohorts were actually prevalent users, and the 12-month washout period resulted in 30% misclassified. Using common washout periods of 6-12 months may insufficiently address prevalent user bias in large commercial claims databases.
authors.affiliation: Division of Pharmaceutical Outcomes & Policy, UNC Eshelman School of Pharmacy, NC, USA.
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Roberts
authors.forename: Stacie B
authors.initials: SB
authors.lastname: Dusetzina
authors.forename: Joel F
authors.initials: JF
authors.lastname: Farley
mesh_terms: D000293:Adolescent; D000328:Adult; D000928:Antidepressive Agents / Q000008:administration & dosage; D057186:Comparative Effectiveness Research; D016208:Databases, Factual; D005260:Female; D006801:Humans; D007004:Hypoglycemic Agents / Q000008:administration & dosage; D000960:Hypolipidemic Agents / Q000008:administration & dosage; D007345:Insurance Claim Review; D008297:Male; D008875:Middle Aged; D017891:Pharmacoepidemiology / Q000379:methods*; D012107:Research Design; D013997:Time Factors
chemical_list: D000928:Antidepressive Agents; D007004:Hypoglycemic Agents; D000960:Hypolipidemic Agents
keywords: comparative effectiveness research; incident user design; methods; pharmacoepidemiology; research design; secondary databases; selection bias