Medication adherence for resistant hypertension: Assessing theoretical predictors of adherence using direct and indirect adherence measures.

OBJECTIVES: This study examined theoretical predictors of long-term medication adherence (i.e., treatment-related beliefs, coherence of beliefs from experience with medication, habit strength, and pill burden) for patients with apparent treatment-resistant hypertension in primary care, using a composite adherence score derived from direct and indirect measures (i.e., prescription refill, self-report, and bioanalytical assays of urine).

DESIGN: Cross-sectional study.

METHODS: Individual patient records were screened for prescription refill adherence. Patients provided a urine sample for adherence screening and completed a battery of psychometric scales, including two self-report adherence measures (N = 204). Convergence of adherence measures was assessed, a composite adherence score was calculated, and hierarchical multiple regression was used to examine the role of theoretical predictors of adherence.

RESULTS: Non-adherence estimates ranged from 20.3 to 41.1%, depending on the assessment method used. Associations among adherence measures were weak to moderate (ρ = .00-.53). Medication-taking habit strength was the strongest predictor of adherence, explaining 19% incremental variance in adherence beyond treatment-related beliefs. Beliefs and coherence did not predict adherence, even for patients with weaker habits. Pill burden was not associated with habit strength or adherence for this sample.

CONCLUSIONS: Associations among unique adherence measures were weak overall, providing further evidence that multiple measures are necessary to accurately assess adherence. Habit strength is a key predictor of adherence for chronic conditions. Both habit strength and pill burden represent important intervention targets for improving long-term medication adherence. Longitudinal inception studies are needed to properly test Common-Sense Model propositions and elucidate the role of beliefs, coherence, and habits in predicting adherence at various stages of the chronic illness trajectory. Statement of contribution What is already known on this subject? Non-adherence to antihypertensives is a leading cause of apparent treatment-resistant hypertension (aTRH). Behaviour maintenance (vs. initiation) factors may be more predictive of long-term adherence. What does this study add? Associations among direct and indirect measures of adherence are generally weak. Habit strength is the strongest predictor of long-term adherence for aTRH in primary care. Inception studies are needed to further validate Common-Sense Model propositions.