Whether level of knowledge of anticoagulation (AC) among patients on warfarin plays a role in maintenance of therapeutic INR or in warfarin-related adverse events is controversial. Most studies conducted on this subject had small patient sample sizes and did not use validated questionnaires to assess patients’ knowledge of AC.


To use the validated Oral Anticoagulation Knowledge (OAK) test (Zeolla MM, 2006) to assess knowledge of AC among patients attending a busy AC clinic, and to examine associations between level of knowledge, INR control and adverse events. We hypothesized that patients with higher OAK test scores (i.e. greater knowledge) would have better INR control (primary outcome) and fewer bleeding and thrombosis events (secondary outcomes).


Consecutive patients who had been followed in our AC clinic (tertiary care, university-affiliated hospital, 20,000 patient-visits per year) for at least one year and consented to participate were asked to complete the OAK test. The OAK test is a 20-question multiple-choice questionnaire that assesses patients’ knowledge of warfarin AC. A passing score is ≥15 correct responses. Patient charts were reviewed to obtain data on clinical and demographic characteristics, and information on INR values and any thrombosis or bleeding events during the preceding 1 year period. Associations between OAK scores and patient characteristics, INR control and bleeding/thrombosis events were assessed by chi-square and t-tests, as appropriate.


Among 252 patients screened for participation, 225 met the inclusion criteria and completed the OAK test. Mean (SD) age was 70 (13.4) years, 53% were male and 75% were on warfarin for >3 years. Indications for AC were atrial fibrillation in 65%, VTE in 8%, mechanical heart valve in 10%, and other in 19%. The mean OAK score was 12/20, and 64% failed the OAK test. Predictors of a pass score on the OAK test were younger age (p= 0.01) and higher level of education (p=0.03). Over the preceding year, 57.3% of INRs were therapeutic, 25.1% subtherapeutic and 17.4% supratherapeutic, and there were 22 bleeding events and 5 thrombosis events. There was no association between OAK score and INR control, or OAK score and bleeding or thrombosis events.


To our knowledge, this is the first study to use the validated OAK test to assess patients’ AC knowledge. We found that younger and more educated patients were more likely to pass the OAK test; however, OAK test result did not predict INR control or occurrence of bleeding or thrombotic events. The OAK test may not be sensitive enough to capture the standard of care practiced in different anticoagulation clinics (e.g. differences in teaching material, frequency of INR checks in stable patients). Also, for some patients, AC knowledge among their caretakers may be more important than self-knowledge. Further research is needed to assess the relationship between AC knowledge, INR control and adverse clinical outcomes.


No relevant conflicts of interest to declare.

Author notes


Asterisk with author names denotes non-ASH members.