16th May 2025

Warfarin

Warfarin is an anticoagulant that acts by inhibiting the synthesis of vitamin K dependent clotting factors. The therapeutic action of warfarin is monitored through an International Normalised Ratio (INR). The INR is derived from prothrombin time (PT) which is calculated as the ratio of the patient’s PT to a standardised PT. The INR which is required and is dependent on the indication with most requiring a target of 2.5 (range 2-3) e.g., atrial fibrillation (AF), deep vein thrombosis. (DVT), and pulmonary embolism (PE). Those with recurrent DVT or PE whilst anticoagulated with an INR above 2 require a higher target of 3.5 (range 3-4). Patients with mechanical heart valves also require anticoagulation, the recommended INR can depend on the location of the valve and patient risk factors (e.g. AF, previous arterial thromboembolism, and mitral stenosis). Those with risk factors and valves in mitral, tricuspid, or pulmonary positions require higher INR targets.

When initiating warfarin therapy INR should be monitored daily or on alternate days, once the INR has stabilised it can be monitored at longer intervals. Closer INR monitoring may be required if there are changes in a patient’s condition, particularly associated with hepatic dysfunction, concurrent illness, or additions of new medications.

It is worth noting that several medications and foods interact with warfarin which may require a change in warfarin dose and INR monitoring frequency. Medications can either increase or decrease the therapeutic effectiveness of warfarin through interactions with cytochrome P450 enzymes or through synergistic pharmacological effects. Food and drugs that increase the effect of warfarin include alcohol, amiodarone, antidepressants, antiplatelets, cranberry juice, and non-steroidal anti-inflammatory drugs. Those that decrease the effect of warfarin include antiepileptics, rifampicin, St John’s Wort, and vitamin K containing complexes e.g., certain enteral feeds and green vegetables.