OBJECTIVES: Approximately 1/3 of patients with partial-onset seizures (POS) are drug resistant and frequently require adjunctive treatment (Brodie 2012).  Although adjunctively-treated patients are identified in claims data by the concurrent administration of two or more AEDs, the minimum AED co-administration period has not been established.  In order to avoid confounding in claims data analyses, the titration time to maintenance dose of AEDs used in adjunctive therapy needs to be carefully weighed in the classification of adjunctively-treated patients. This study sought to identify the impact of different co-administration periods on the proportion of patients identified as receiving adjunctive therapy within claims data.  METHODS: This retrospective study used the HCMS employee database of commercially insured subjects. Subjects with POS [ICD9-CM=345.4x,345.5x] between Jan-2001 and Jun-30-2014 were identified from the employee’s spouses and assigned an index date based on their initial AED use.  Subjects were required to have >365 days of continuous eligibility following their index date. Patients were classified as having adjunctive therapy on the basis of having at least 30, 45, 60, or 90 days of concomitant AED use in the year following the index date. RESULTS: