Inclusion Criteria- Patients on long-term warfarin (for prophylaxis of arterial embolism in patients with atrial fibrillation or mechanical heart valve replacement, or secondary prophylaxis after VTE) with a target INR of 2.0-3.0 or 2.5-3.5,
- Anticoagulant therapy managed by the clinic (HHS - General Hospital) for at least 6 months prior to enrolment, and
- Maintenance dose of warfarin unchanged for the previous 6 months or longer.
Exclusion Criteria- Age <18 years,
- Life expectancy of less than 1 year,
- Attending physician believes the patient is not suitable for the study (e.g. psychiatric disorder, history of non-compliance),
- Geographic inaccessibility or
- Failure to obtain written consent.