Prevalent aetiologies of non-therapeutic warfarin anticoagulation in a network of pharmacist-managed anticoagulation clinics

Abstract

There is wide inter-patient and intra-patient variability in the pharmacodynamic profile of warfarin. To determine the prevailing aetiologies of non-therapeutic warfarin anticoagulation episodes among patients currently enrolled in an outpatient anticoagulation clinic and compare the relative frequency in which they occur compared to therapeutic anticoagulation regimens. Prospective, observational cohort study set within three pharmacist-managed anticoagulation clinics in a community outpatient health system. Patients were included, if they were seen for an office visit during the 6-month period from September 2006 to March 2007 and evaluated for the presence or absence of 12 investigational factors linked to non-therapeutic anticoagulation results. Multivariate stepwise logistic regression performed to assess predictive value of each factor. A total of 5817 patient-visits were documented producing 2886 (49.6%) non-therapeutic and 2931 (50.4%) therapeutic International Normalized Ratio (INR) readings. The most prevalent aetiologies linked to non-therapeutic INR results included change in dietary vitamin K intake (16.9%, OR 6.4), non-compliance (15.0%, OR 4.9), and initiation of anticoagulant therapy (9.9%, OR 2.3). The factor with the highest predictive value of non-therapeutic INR results was a change in health status (OR 9.5) despite its lower rate of frequency (4.9%). Despite identification of many causative factors in this study, 40.2% of non-therapeutic INR readings had no known aetiology. In the end, the lack of any study factor was a greater predictor of therapeutic anticoagulation (86.2%), than the presence of a study factor was for predicting non-therapeutic INR values (51.4%). Change in health status was the strongest predictor of non-therapeutic INR levels out of the investigational factors evaluated. Our study demonstrated that there are many aetiologies for non-therapeutic INR values that were not explained by our investigational factors.