• medication adherence;
  • MEMS;
  • warfarin;
  • anticoagulation



Warfarin is widely used to prevent stroke and venous thromboembolism despite its narrow therapeutic window. Warfarin nonadherence is a substantial problem, but risk factors have not been well elucidated.


A prospective cohort study of adults initiating warfarin at two anticoagulation clinics (University and VA-affiliated) was performed to determine factors affecting nonadherence to warfarin. Nonadherence, defined by failure to record a correct pill bottle opening each day, was measured daily via electronic medication event monitoring systems (MEMS) caps. A multivariable explanatory model using logistic regression for longitudinal data was used to identify risk factors for nonadherence.


One hundred eleven subjects were followed for a median of 137 days. Warfarin nonadherence was common (4787 of 22 425 or 21% of patient-days observed). Factors independently associated with higher odds of nonadherence included education beyond high school (odds ratio (OR) 1.8 (95%CI 1.2–2.7)), lower Short Form (SF)-36 mental component score (OR 1.4 (1.1–1.6) for each 10 point decrease); and impaired cognition (≤19 points) on the Cognitive Capacity Screening Examination (CCSE) (OR 2.9 (1.7–4.8)). Compared to currently employed subjects, unemployed (OR 0.6 (0.3–1.2)) and retired (OR 0.5 (0.3–0.8)) subjects had somewhat improved adherence; disabled subjects over age 55 had worse adherence (OR 1.8 (1.1–3.1)) than younger disabled subjects (OR 0.8 (0.4–1.5)).


Poor adherence to warfarin is common and risk factors are related to education level, employment status, mental health functioning, and cognitive impairment. Within the carefully controlled anticoagulation clinic setting, such patient-specific factors may be the basis of future interventions to improve nonadherence. Copyright © 2008 John Wiley & Sons, Ltd.