Antiparkinson drug use and adherence in medicare part D beneficiaries with Parkinson's disease.

BACKGROUND: Antiparkinson drugs (APDs) are the mainstay of managing Parkinson's disease (PD). However, there is paucity of evidence documenting patterns of APD use and examining factors associated with adherence to APDs.
OBJECTIVES: Our goal was to provide updated, comprehensive population-based data on APD use and adherence and to examine characteristics associated with adherence behaviors.
METHODS: We analyzed data from 7583 beneficiaries with PD who had ≥ 2 APD prescription fills and were continuously enrolled in Medicare Parts A, B, and D for up to 19 months (from June 1, 2006, through December 31, 2007) or until death in 2007. We assessed 5 patterns of APD use: (1) concurrent use of ≥ 2 APD classes for ≥ 30 days; (2) switching of APDs from 1 to another; (3) augmentation of the existing regimen with a new APD; (4) duration of therapy, defined as days of APD treatment; and (5) adherence measured by using the medication possession ratio (MPR). We corrected for sample selection bias inherent in patients' self-selection into either a Part D plan or a Medicare Advantage Prescription Plan by using Heckman's 2-stage procedures.
RESULTS: APD users were pre-dominantly aged ≥ 65 years (93.6%), female (59.9%), and white (89.3%). Almost one half (43.2%) of APD users concurrently used ≥ 2 APD classes. One in 4 APD users experienced changes in their APD regimen, with 16.4% switching medications and 21.1% augmenting their current regimen. Three quarters of APD users had therapy lasting ≥ 436 days (75.3%) and an MPR ≥ 0.8 (72.7%). Multivariate analyses revealed that patients aged ≥ 65 years, of non-white race, non-low-income subsidy recipients, late Part D enrollees, cognitively impaired, highly comorbid, and who experienced multiple changes in APD therapy were less likely to adhere to APD therapy. We were able to generalize our findings to all Part D enrollees by correcting for sample selection bias using the Heckman approach. These population-level, generalizable observations provide better understanding of APD use and adherence and assist in the design of interventions for poor adherence. Limitations include cross-sectional study design and constraints in administrative data that preclude measurement of other potential factors related to adherence.
CONCLUSIONS: A substantial proportion of these Medicare beneficiaries with PD used multiple APDs concurrently, experienced switching and/or augmentation of APDs, and had poor adherence to APDs. Patient characteristics and clinical and drug-related factors were important predictors of APD adherence.