The conventional approach in measuring treatment persistence tended to use only the first prescription episode even though some patients received multiple prescriptions of the same medication. In this study, we assessed the extent to which patients received multiple prescriptions and levels of treatment persistence associated with each prescription episode.

Using 2000-2004 data from the Veterans Health Administration in the United States, we identified patients with schizophrenia using ICD-9-CM codes and defined initiation of the target agent as 6-month "clean" period of no target drugs prior to initiation and reserved one year following the initiation to calculate treatment persistence, or time to discontinuation, as defined by a gap of >15 or > 30 days.

The study found that about 25% of the patients had two or more treatment episodes, and that the levels of treatment persistence exhibited variation for patients with one, two, or three prescriptions. Generally speaking, among patients with one prescription, initiators of typical agents tended to fare worst in the level of treatment persistence. This finding suggests that conventional approach in calculating treatment persistence tends to underestimate the gap between typical and atypical agents.

Considering that patients with different number of treatment episodes might differ in disease profiles, this treatment episode-specific approach offered a fair comparison of the levels of treatment persistence across patients with different number of treatment episodes.