Vitamin K antagonists (VKAs) have been used as anticoagulants for over 50 years, because vitamin K is an essential part of the coagulation process. Warfarin is the most commonly used VKA, although other coumarin derivatives are also used. VKAs have many limitations and are difficult to manage — studies show that fewer than half of patients using warfarin are within the therapeutic range in the community setting. Because of the variability in the dose–response with VKA medications, monitoring the degree of anticoagulation is essential. The international normalized ratio (INR) provides a standardized measure of the VKA anticoagulant effect.