Warfarin

Key points

• Inhibits reduction of vitamin K to its active hydroquinone form → cofactor for carboxylation of clotting factors II, VII, IX, X (mnemonic 1972) and protein C
• Targets: VTE INR 2.5 (recurrent 3.5) · atrial fibrillation 2.5 · mechanical valves depend on valve type/location (mitral generally higher than aortic)
• Monitored by INR (patient prothrombin time ÷ normal); long half-life — stable INR may take several days
• Potentiated by liver disease, P450 inhibitors (amiodarone, ciprofloxacin), cranberry juice and NSAIDs (displace from albumin, inhibit platelets)
• Side-effects: haemorrhage · teratogenic (safe in breastfeeding) · skin necrosis (early protein C fall → temporary procoagulant state, avoided by concurrent heparin) · purple toes