British Journal of Health Psychology (2018) © 2018 The British Psychological Society www.wileyonlinelibrary.com Medication adherence for resistant hypertension: Assessing theoretical predictors of adherence using direct and indirect adherence measures Hannah Durand 1 * , Peter Hayes 2 , Brendan Harhen 3 , Ann Conneely 4 , David P. Finn 4 , Monica Casey 2 , Andrew W. Murphy 2,5 and Gerard J. Molloy 1 1 Medication Adherence Across the Lifespan (MEDAL) Group, School of Psychology, National University of Ireland, Galway, Ireland 2 Discipline of General Practice, School of Medicine, National University of Ireland, Galway, Ireland 3 National Centre for Biomedical Engineering Science, National University of Ireland, Galway, Ireland 4 Discipline of Pharmacology and Therapeutics, School of Medicine, National University of Ireland, Galway, Ireland 5 Health Research Board Primary Care Clinical Trials Network, Galway, Ireland 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). Conver- gence 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 (q = .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 *Correspondence should be addressed to Hannah Durand, MSc, G050 AMBE, School of Psychology, National University of Ireland, Galway H91 EV56, Ireland (email: h.durand1@nuigalway.ie). DOI:10.1111/bjhp.12332 1