Revisiting the washout period in the incident user study design: Why 6-12 months may not be sufficient

Aims: The purpose of this study was to describe how washout period duration affects the size and accuracy of retrospective incident user cohorts. Materials & 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. Results & conclusion: 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.