Medicare Part D is health insurance for people who are either disabled or over 65 years old or suffer from End Stage Renal Disease. It provides coverage for outpatient prescription drugs and is a departure from the traditional Medicare program in which the benefits are uniform. Coverage has a minimum drug formulary packaged in standard plans or actuarially equivalent plans that offers varying additional benefits.

Objective: (1) to examine the effects of coverage on utilization of select antipsychotic drugs: olanzapine, quetiapine, risperidone, and haloperidol, (2) to understand out-of-pocket expenditure patterns among beneficiaries with schizophrenic and bipolar disorders in Washington State. Design: a descriptive and analytical cross-sectional, retrospective study design that used 2008 Part D Event data (1922 unique individuals). Measurements: utilization of four antipsychotics (in number of claims and medication adherence measured in Proportion of Days Covered) and out-of-pocket expenditures before the gap-in-coverage period (pre-ICL), during the gap-in-coverage period (ICL), and post gap-in-coverage period (post-ICL). Results: Beneficiaries who experienced all three benefit phases (i.e. pre-ICL, ICL, and post-ICL) comprised a very small percentage, 1.36 percent, of the study population and generated only 141 claims. Among these beneficiaries, 70.92 percent had low PDC adherence that primarily occurred during the ICL period (35.46 percent), followed by the pre-ICL period (21.3 percent), and the post-ICL period (14.19 percent). Only 29 percent of beneficiaries who experienced all three benefit phases had high PDC, with the highest percentage (13.5 percent) occurring in the post-ICL period (13.48 percent), followed by the pre-ICL period (11.35 percent) and the ICL period (4.26 percent). Beneficiaries spent far more in average our-of-pocket expense during the gap period ($36.43) than averages in the pre-gap period and the catastrophic coverage period combined ($14.19). Non-adherent beneficiaries paid a greater average than adherent beneficiaries: low PDC beneficiaries paid an average of $27.95 and a median of $2.25, while high PDC beneficiaries paid an average of $7.50 and a median of $ 0. Conclusion: Utilization of antipsychotics is affected by the extent of Part D plan coverage beneficiaries choose and elements of Part D plans effect medication utilization differently.