Summary

Background: Patients receiving long-term warfarin frequentlydevelop asymptomatic excessive prolongation of their internationalnormalized ratio (INR) results. The most appropriate managementstrategy in these patients is unknown. This prospective cohort studywas designed to address whether 1 mg of oral vitamin K effectivelyreduces the INR value of such patients. Methods: A prospective cohortstudy was performed in two tertiary care teaching hospitals, in which62 patients receiving warfarin who had INR values between 4.5 and10.0 received 1 mg of oral vitamin K. All patients had daily INR valuesand clinical assessments performed. Results: The mean INR value atpresentation was 5.79 (95% confidence interval (CI) 5.48 to 6.09,range 4.5 to 9.5). Sixteen hours after receiving the 1 mg of oral vitaminK, the mean INR was 2.86 (95% CI 2.50 to 3.23). On the second andthird days after vitamin K, the mean INR values were 2.20 (1.93 to2.47) and 2.14 (1.85 to 2.44), respectively. No adverse events orbleeding complications were observed. In three patients (6%) the INRvalue rose between the time of vitamin K administration and the nextINR determination; two patients received a further 2 mg dose ofsubcutaneous vitamin K. Conclusions: In patients receiving warfarinwho have asymptomatic excessive prolongations in their INR results,1 mg of oral vitamin K reliably reduces the INR to the therapeuticrange within 24 h. This therapy is more convenient, less expensive, andmight be safer than parenteral vitamin K. Thus, it should be consideredin all non-bleeding patients receiving warfarin, who present with INRresults of 4.5 to 9.5.