Comparison of 5-mg and 10-mg Loading Doses in Initiation of Warfarin Therapy

Background: Loading doses of warfarin that are larger than those used for maintenance therapy are widely used in clinical practice, but they have never been prospectively evaluated. Objective: To compare the effect of 5- and 10-mg loading doses of warfarin on laboratory markers of warfarin's anticoagulant effect. Design: Randomized clinical trial. Setting: Tertiary care teaching hospital. Patients: 49 patients seen over a 5-month period with a target international normalized ratio (INR) of 2.0 to 3.0. Intervention: Patients were randomly assigned to receive an initial dose of 5 or 10 mg of warfarin. Subsequent doses of warfarin were administered on the basis of dosing nomograms. Measurements: INRs and levels of factors II, VII, IX, and X and protein C were measured daily for 5 days. Results: 11 of 25 patients in the 10-mg group (44% [95% Cl, 34% to 54%]) and 2 of 24 patients in the 5-mg group (8% [Cl, 3% to 14%]) had INRs greater than 2.0 at 36 hours (P = 0.005), at which time the factor VII levels were 27% (Cl, 18% to 36%) in the 10-mg group and 54% (Cl, 43% to 65%) in the 5-mg group (P 0.2). At 60 hours, 9 of 25 patients in the 10-mg group (36% [Cl, 17% to 54%]) and no patients in the 5-mg group had INRs greater than 3.0. At 84 hours, 15 of 24 patients in the 10-mg group (63% [Cl, 43% to 81%]) and 19 of 24 patients in the 5-mg group (79% [Cl, 62% to 95%]) had INRs between 2.0 and 3.0. Four patients in the 10-mg group and 1 patient in the 5-mg group received vitamin K for excessive prolongation of the INR. Conclusions: A 5-mg loading dose of warfarin produces less excess anticoagulation than does a 10-mg loading dose; the smaller dose also avoids the development of a potential hypercoagulable state caused by precipitous decreases in levels of protein C during the first 36 hours of warfarin therapy.