Association Between Patient Knowledge of Anticoagulation, INR Control, and Warfarin-Related Adverse Events

Background: Whether the level of patient's knowledge about warfarin plays any role in maintenance of therapeutic international normalized ratio (INR) is controversial. Several studies have looked at patients' warfarin knowledge and the level of patients' anticoagulation control (AC). Most studies had small numbers and did not use validated questionnaires. Objectives: To use the Oral Anticoagulation Knowledge (OAK) test to assess patients' knowledge of AC and to examine associations between knowledge, INR, and adverse events. Methods: In this cross-sectional study, patients were asked to complete the OAK test. Data on clinical and demographic characteristics, INR values, and thrombosis or bleeding events during the preceding 1 year period were collected. Associations between OAK scores, patient characteristics, proportion of therapeutic INRs, and bleeding/thrombosis events were assessed. Results: A total of 225 patients completed the OAK test. Mean (SD) age was 70 (13.4) years, 53% were male, and 75% were on warfarin for >3 years. Over the preceding year, 57.3% of INRs were therapeutic, and there were 22 bleeding and 6 thrombotic events. The mean OAK score was 12/20 (passing score = 15/20); 64% of patients failed the OAK test. Predictors of passing the OAK test were younger age (P = .01) and higher level of education (P = .03). There was no association between OAK score and proportion of therapeutic INRs, or OAK score and bleeding or thrombosis events. Conclusion: We used the OAK test to assess patients' AC knowledge. Results suggests that while younger and more educated patients were more likely to pass the OAK test, the OAK test results may not predict INR control or occurrence of bleeding or thrombotic events.