Assessment of an Age-Adjusted Warfarin Initiation Protocol

Abstract

OBJECTIVE: To examine the time taken to reach a stable international normalized ratio (INR), as well as the incidence of overanticoagulation of an age-adjusted warfarin initiation protocol. METHODS: Inpatients and outpatients commencing warfarin therapy at 2 teaching hospitals were dosed according to the age-adjusted protocol. Data were collected prospectively. MAIN OUTCOME MEASURES: Time to reach a stable INR of 2–3 and the number of patients experiencing an INR ≥4 during the first week of warfarin therapy. RESULTS: Seventy-three patients were assessed; at the completion of the 4-day titration protocol, 63% had achieved a stable INR. After an additional 2 days of empiric dosage adjustment by the attending physician, 86% of the subjects demonstrated a stable INR. Five patients (7%) experienced an INR ≥4. These patients had a nonsignificant trend toward a lower plasma albumin level compared with other patients (p = 0.057, Student's t-test). The INR-driven dose adjustments on days 3 and 4 of this protocol coped with other variables that have been shown to affect maintenance warfarin dosing. These included weight, gender, pharmacologic factors affecting clearance, and the presence of certain predesignated risk factors. CONCLUSIONS: The age-adjusted dosing protocol rapidly achieved a stable INR with minimal overanticoagulation. Patients with low serum albumin levels (<3.0 g/dL) may be sensitive to the effects of warfarin during the loading phase.