Summary
Background: Patients receiving long-term warfarin frequently develop asymptomatic
excessive prolongation of their international normalized ratio (INR) results. The
most appropriate management strategy in these patients is unknown. This prospective
cohort study was designed to address whether 1 mg of oral vitamin K effectively reduces
the INR value of such patients. Methods: A prospective cohort study was performed
in two tertiary care teaching hospitals, in which 62 patients receiving warfarin who
had INR values between 4.5 and 10.0 received 1 mg of oral vitamin K. All patients
had daily INR values and clinical assessments performed. Results: The mean INR value
at presentation 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 vitamin K, the mean INR was 2.86
(95% CI 2.50 to 3.23). On the second and third days after vitamin K, the mean INR
values were 2.20 (1.93 to 2.47) and 2.14 (1.85 to 2.44), respectively. No adverse
events or bleeding complications were observed. In three patients (6%) the INR value
rose between the time of vitamin K administration and the next INR determination;
two patients received a further 2 mg dose of subcutaneous vitamin K. Conclusions:
In patients receiving warfarin who have asymptomatic excessive prolongations in their
INR results, 1 mg of oral vitamin K reliably reduces the INR to the therapeutic range
within 24 h. This therapy is more convenient, less expensive, and might be safer than
parenteral vitamin K. Thus, it should be considered in all non-bleeding patients receiving
warfarin, who present with INR results of 4.5 to 9.5.
Dr. Crowther is the recipient of a Medical Research Council of Canada Research Fellowship.
Dr. Ginsberg is a Career Investigator of the Heart and Stroke Foundation of Ontario.
