Measuring medication adherence in older community-dwelling patients with multimorbidity

Older people with several chronic conditions require multiple drugs from different classes to be adequately treated. This study aims to: (i) measure medication adherence across multiple conditions and therapeutic drug groups in older community-dwelling patients, and (ii) examine the effect of multimorbidity on adherence. This is a retrospective cohort study of medication adherence in 855 community-dwelling patients aged >= 70 years from 15 practices in Ireland using the Health Service Executive Primary Care Reimbursement Service (HSE-PCRS) pharmacy claims database. Multimorbidity was measured using the RxRisk-V and by the number of different drug classes. The RxRisk-V algorithm classifies prescription drug fills into 45 chronic disease classes for older populations based on the WHO Anatomical Therapeutic Chemical classification system. Adherence to medications was assessed by: (i) calculating the average medication possession ratio (MPR) per patient and (ii) an MPR < 80%. The overall median MPR for the cohort was 0.83 (IQR 0.69, 0.91). The conditions with the highest MPRs were hypothyroidism (mean MPR = 0.88, SD = 0.20) and type 2 diabetes (mean MPR = 0.83, SD = 0.19), followed by heart disease. On average, 20-40% of patients were non-adherent (MPR < 80%) across all conditions. There was an inverted U-shaped relationship between the mean MPR and number of morbidities and drug classes. Adherence varied per patients' morbidity burden, with higher adherence for certain combinations of chronic conditions. In total, 31% of older patients with multimorbidity were non-adherent to their medication but adherence levels varied across treatment categories and chronic conditions.