ABSTRACT
Warfarin Warfarin works by interfering with vitamin K-dependent gamma-carboxylation
of coagulation proteins II, VII, IX, and X. As a result of warfarin therapy, these coagu lation factors cannot bind calcium. This causes impairment of these factors’ binding to membranes and to fold into proper configuration. Therapy with warfarin is initi ated by giving the patient 5-Ю mg in the evening for the first two nights (2.5 mg in those over 75 years) and adjusting the dose to achieve an adequate prothrombin time. Although the use of a 10 mg loading dose has been traditional in the past, for most people this is too much. Multiple trials show that using a 10 mg loading dose causes one to overshoot and leads to a delay in achieving a stable therapeutic INR. A practi cal approach is to use 5mg in loading patients over the age of 50 or in patients with albumin under three and 10 mg in other patients. The elderly patient (over age 75) may only need a 2.5 mg loading dose. Nomograms for 5 and 10 mg warfarin loading doses are given in Table 24.1. The effect of warfarin on the INR takes 36 hours to occur so the morning INR reflects the effect of the warfarin dose 36 hours before.
