Longitudinal medication adherence group-based trajectories of aging adults in the US: A retrospective analysis using monthly proportion of days covered calculations.

BACKGROUND: It is thought that half of the patients with chronic conditions are not adherent to their medications, which contributes to significant health and economic burden. Many studies estimate medication non-adherence by implementing a threshold of ≥80% of Proportion of Days Covered (PDC), categorizing patients as either adherent or non-adherent. Healthcare quality metrics pertaining to medication use are based on this dichotomous approach of medication adherence, including the Medicare Part D Star Ratings. Among others, the Medicare Part D Star Ratings rewards part D plan sponsors with quality bonus payments based on this dichotomous categorization of beneficiaries' medication adherence.

OBJECTIVES: Describe the longitudinal adherence trajectories of adults ≥65 years of age covered by Medicare for 3 classes of drugs in the Part D Star Ratings: diabetes medications, statins, and select antihypertensives.

METHODS: This study used Medicare healthcare administrative claims data linked to participants from the Health Retirement Study between 2008 and 2016. Group-based trajectory models (GBTM) elicited the number and shape of adherence trajectories from a sample of N = 11,068 participants for the three pharmacotherapeutic classes considered in this study. Medication adherence was estimated using monthly PDC.

RESULTS: GBTM were estimated for the sample population taking antihypertensives (n = 7,272), statins (n = 8,221), and diabetes medications (n = 3,214). The hypertension model found three trajectories: high to very high adherence (47.55%), slow decline (32.99%), and rapid decline (19.47%) trajectories. The statins model found 5 trajectories: high to very high adherence (35.49%), slow decline (17.12%), low then increasing adherence (23.58%), moderate decline (12.62%), and rapid decline (11.20%). The diabetes medications model displayed 6 trajectories: high to very high adherence (24.15%), slow decline (16.84%), high then increasing adherence (25.56%), low then increasing (13.58%), moderate decline (10.60%), and rapid decline (9.27%).

CONCLUSIONS: This study showed the fluid nature of long-term medication adherence to the medications considered in the Medicare Part D Star Ratings and how it varies by pharmacotherapeutic class. These challenge previous assumptions about which patients were considered adherent to chronic medications. Policy and methodological implications about medication adherence are discussed.