The least frequently used mechanism for monitoring warfarin therapy is patient self-monitoring using a point of care testing (POCT) device. Patients monitored with POCT are responsible for determining their INR and adjusting their warfarin dose based on their INR result. Patients who self-monitor using point of care testing and who find themselves to have an INR of more than 4.5 will be asked to contact their clinic staff. Prior to being approached for consent to enter this study the patient's INR will be confirmed on a venous blood sample organized by their host anticoagulant clinic. Given that patients in this group will be randomized while at the study centre, for the purposes of this study such patients will be considered to be in group 1.