Warfarin dose assessment every 4 weeks versus every 12 weeks in patients with stable international normalized ratios: a randomized trial.

Background: Guidelines recommend that patients receiving warfarin undergo international normalized ratio (INR) monitoring every 4 weeks.

Objective: To investigate whether assessment of warfarin dosing every 12 weeks is as safe as assessment every 4 weeks.

Design: Noninferiority randomized trial. The randomization schedule (in a 1:1 ratio) was computer-generated, and allocation was concealed until the database was locked by using a centralized schedule. Patients, study and clinical personnel, adjudicators of clinical events, and the study statistician were blinded to treatment assignment. (ClinicalTrials.gov registration number: NCT00356759) SETTING: Single center in Hamilton, Ontario, Canada.

Patients: 250 patients receiving long-term warfarin therapy, whose dose was unchanged for at least 6 months; 226 completed the study.

Intervention: Dosing assessment every 12 weeks (n = 124) compared with every 4 weeks (n = 126) for 12 months. Patients in the 12-week group were tested every 4 weeks; sham INRs within the target range were reported for two of the three 4-week periods.

Measurements: Percentage of time in the therapeutic range (primary outcome) and number of extreme INRs, changes in maintenance dose, major bleeding events, objectively verified thromboembolism, and death (secondary outcomes).

Results: The percentage of time in the therapeutic range was 74.1% (SD, 18.8%) in the 4-week group compared with 71.6% (SD, 20.0%) in the 12-week group (absolute difference, 2.5 percentage points [1-sided 97.5% upper confidence bound, 7.3 percentage points]; noninferiority P = 0.020 for a 7.5-percentage point margin). Fewer patients in the 12-week group than in the 4-week group had any dose changes (37.1% vs. 55.6%; absolute difference, 18.5 percentage points [95% CI, 6.1 to 30.0 percentage points]; P = 0.004). Secondary outcomes did not differ between groups.

Limitations: Patients in the 12-week group had testing and contact with clinic staff every 4 weeks. The study was conducted at a single center and used surrogate outcomes.

Conclusion: Assessment of warfarin dosing every 12 weeks seems to be safe and noninferior to assessment every 4 weeks. A comparison of INR testing, patient contact, and warfarin dose assessment every 12 weeks versus every 4 weeks is necessary before INR testing every 12 weeks can be routinely recommended for practice.

Primary Funding Source: Physicians' Services Incorporated Foundation.