Management and dosing of warfarin therapy

When initiating warfarin therapy, clinicians should avoid loading doses-tha t can raise the International Normalized Ratio (INR) excessively; instead, warfarin should be initiated with a 5-mg dose (or 2 to 4 mg in the very eld erly). With a 5-mg initial dose, the INR will not rise appreciably in the f irst 24 hours, except in rare patients who will ultimately require a very s mall daily dose (0.5 to 2.0 mg). Adjusting a steady-slate warfarin dose dep ends on the measured INR values and clinical factors: the dose does not nee d to be adjusted for a single INR that is slightly out of range, and most c hanges should alter the total weekly dose by 5% to 20%. The INR should be m onitored frequently (eg, 2 to 4 times per week) immediately after initiatio n of warfarin; subsequently, the interval between INR tests can be lengthen ed gradually (up to a maximum of 4 to 6 weeks) in patients with Stable INR values. Patients who have an elevated INR will need more frequent testing a nd may also require vitamin K1. For example, a nonbleeding patient with an INR of 9 can be given low-dose vitamin K1 (eg, 2.5 mg phytonadione, by mout h). Patients who have an excessive INR with clinically important bleeding r equire clotting factors leg, fresh-frozen plasma) as well as vitamin K1. Am J Med. 2000;109:481-488. (C) 2000 by Excerpta Medica, Inc.