Approaches to optimal dosing of vitamin K antagonists

Abstract

Although vitamin K antagonists (VKAs) such as warfarin have been used clinically for decades, evidence supporting how best to manage their use in clinical practice is lacking, but continues to emerge. This article summarizes available information regarding the clinical management of VKAs with focus on dosing strategies. For patients with previously stable international normalized ratio (INR) control, the single mildly out-of-range INR does not warrant a change in VKA dose. For out-of-range INRs, prompt repeat testing is associated with better INR control. After the first or second in-range INR value a maximum recall interval of 28 days is optimal, but after the third or greater consecutive in-range INR, longer recall intervals (up to 12 weeks for very stable patients) can be used. The use of validated VKA dosing nomograms is suggested as a means of reducing unwanted variability in VKA dosing decisions. Ensuring timely INR monitoring, and adjusting VKA doses when necessary, is important when interacting medications are prescribed during VKA therapy. Daily low-dose vitamin K supplementation is unlikely to improve INR control in patients with stable INR control but may be of benefit in VKA patients with unexplainable variability in the INR response. Dosing decisions during VKA therapy should follow a systematic and coordinated process as used in dedicated anticoagulation management services. Patient self-management of VKA therapy offers advantages for motivated patients who can demonstrate competency in self-management including fingerstick INR testing. Most patients with excessive anticoagulation who are not bleeding can be managed without administering vitamin K. There is an ongoing need for research evaluating VKA dosing practices that can consistently improve the outcomes of VKA therapy.