Measuring medication adherence in older community-dwelling patients with multimorbidity

Purpose: 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. Methods: 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%. Results: 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. Conclusion: In total, 31% of older patients with multimorbidity were non-adherent to their medication but adherence levels varied across treatment categories and chronic conditions.