The purpose of this study is to determine if daily Vitamin K supplementation stabilizes INRs in patients taking warfarin with a history of frequent dose changes or variable INRs.

Consistent daily Vitamin K intake may contribute to the stabilization of INR values in patients with unstable INRs on warfarin therapy. Subjects with unstable INRs will be identified from the existing patient records of the UNC Anticoagulation Clinics. Eligible subjects will sign an informed consent form and will receive a point-of-care (POC) INR monitoring device. They will undergo 9 weeks of home INR monitoring via the POC device (with monthly clinic follow-up). They will subsequently be provided with 500 mcg Vitamin K tablets to be taken once daily. Weekly POC INR monitoring with monthly clinic follow-up will continue for an additional 8 weeks.

Inclusion Criteria:

• Patient of UNC Anticoagulation Clinic with INR target of 2.0-3.0 and at least 9 months on uninterrupted warfarin
• Unstable INRs for the past 6 months, defined as a minimum number of warfarin dose changes of 3
• Patient demonstration of proper POC device technique on 2 measurements during one teaching session with investigator
• Age greater than or equal to 18 years

Exclusion Criteria:

• Stroke or TIA within the previous 12 months
• VTE within the last 3 months
• Antiphospholipid antibody syndrome
• Pregnant patients, since warfarin is contraindicated during pregnancy