Investigating the association between medication adherence and health-related quality of life in COPD: Methodological challenges when using a proxy measure of adherence.

BACKGROUND: The association between non-adherence to medication and health-related quality-of-life (HRQoL) in Chronic Obstructive Pulmonary Disease (COPD) remains poorly understood. Different ways to deal with methodological challenges to estimate this association have probably contributed to conflicting results.

AIM: To investigate the association between medication adherence and HRQoL, thereby illustrating methodological challenges that need to be addressed.

METHODS: We used longitudinal patient-level data from a cluster-randomized controlled trial (i.e. RECODE) including three-year data on type and dose of COPD maintenance medication prescribed and HRQoL (Clinical COPD Questionnaire [CCQ], st. George Respiratory Questionnaire [SGRQ], EuroQol 5-dimensions [EQ-5D]) of 511 patients. A linear mixed model was used to assess the association between adherence and HRQoL using a fixed cut-off of 80% of the proportion of days covered (PDC) to define adherence. Subsequently, we investigated the impact of differences in disease severity; lifestyle; and reversed causality, representing the methodological challenges. Additionally, we investigated the impact of changing the definition of adherence.

RESULTS: In unadjusted analyses, and analyses adjusting for demographic characteristics only, SGRQ score was worse in the adherent compared to the non-adherent group. This association disappeared when correcting for disease severity and/or lifestyle. A better SGRQ score was predictive of decreased adherence in the following year. However, accounting for the previous HRQoL did not result in positive associations between adherence and HRQoL. When defining four categories of adherence, patients with a PDC between 80 and 99% had a significantly worse SGRQ score compared to patients with a PDC <60%, even after correction for lifestyle. There was no significant association between adherence and CCQ or EQ-5D.

CONCLUSION: This study showed persistent methodological challenges in the investigation of the effect of medication adherence on HRQoL in COPD. A positive association of adherence and HRQoL was not found, even after adjusting for lifestyle, disease severity, and previous HRQoL.