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Some on Warfarin May Monitor Less Frequently

A comparison of testing and contact is necessary before adopting in clinical practice

Assessing warfarin dosing every 12 weeks is safe and noninferior to assessment every four weeks, according to a study published recently in the Annals of Internal Medicine.

Anticoagulant treatment with vitamin K antagonists requires frequent prothrombin time (PT) testing and dose adjustment. Recommendations regarding the frequency of such testing differ, however, and the authors hypothesized that for patients with stable PT results, less frequent testing might be safe and feasible. They thus compared PT testing and dose assessment every four weeks versus every 12 weeks over one year in patients whose maintenance dose had been unchanged in the previous six months.

Between November 16, 2006 and December 19, 2008, the researchers randomly assigned 250 patients to receive PT testing every four weeks or every 12 weeks. Patients were eligible if they were receiving long-term warfarin treatment with a therapeutic international normalized ratio (INR) range of 2.0 to 3.0, or 2.5 to 3.5 if they had been managed at the authors' clinic for at least six months before enrollment, and if their maintenance dose of warfarin had been unchanged for at least six months. The primary outcome was the percentage of time in the therapeutic range (TTR). Secondary outcomes were extreme INRs, changes in maintenance dose, major bleeding events according to published criteria, objectively verified thromboembolic events, and deaths. Mean TTR percentage was 74.1 in the four-week group and 71.6 for the 12-week group. Fewer patients in the 12-week group had any dose changes. Secondary outcomes did not differ between the groups. The authors conclude that although the results of the current study suggest that testing every 12 weeks is safe and feasible, "a phase 3 trial comparing testing and contact every 4 weeks with every 12 weeks [is] necessary before prolonged intervals for testing and dose assessment can be recommended for clinical practice."

Source: Schulman S, Parpia S, Stewart C, et al. 2011. Warfarin dose assessment every 4 weeks versus every 12 weeks in patients with stable international normalized ratios: a randomized trial. Annals of Internal Medicine 155:653-659.