The major challenge in warfarin therapy is its narrow therapeutic range. However, the full anticoagulant effect may be delayed for 5-7 days due to the long half-life of prothrombin. An effect on the INR generally occurs within 24 hours after drug administration. It is almost fully bound to albumin in blood thus hypoalbuminemic patients (e.g., malnourished, liver disorders, post-operative, etc.) need lower doses. Warfarin is given orally and is absorbed rapidly and completely. 3 Approximately a third of the patients who would benefit from warfarin never receive it, and over half of those who do receive warfarin are managed suboptimally 1, 6 because of the complex pharmacology and numerous drug, disease, dietary and herbal interactions. 4, 5 Studies have shown that warfarin therapy could reduce the risk of stroke in these patients by about 66%. 1-3 Approximately 44,000, or 6%, of British Columbians > 65 years live with AF. Warfarin therapy reduces the risk of thromboembolic events and has demonstrated effectiveness for the prevention of stroke in patients with atrial fibrillation (AF). The series includes three other guidelines: Stroke and Transient Ischemic Attack – Acute and Long-Term Management Atrial Fibrillation – Diagnosis and Management and Use of Non-Vitamin K Antagonist Oral Anticoagulants (NOAC) in Non-Valvular Atrial Fibrillation.

This guideline is part of the BCGuidelines.ca - Stroke and Atrial Fibrillation series. Perioperative management of warfarin is covered in the BCGuidelines.ca - Warfarin Therapy – Management During Invasive Procedures and Surgery.

This guideline assumes the physician has reviewed the indications for warfarin and the duration of therapy as these are not discussed in this guideline. The guideline describes: 1) warfarin initiation, 2) international normalized ratio (INR) monitoring with optimal ranges, and 3) warfarin dosage adjustment. This guideline provides recommendations for the long-term management of warfarin therapy in patients aged ≥19 years in the primary care setting.

Warfarin overdose antidote