A 67-year-old patient, Mrs. Leiden is taking 40 mg of warfarin per week (one 5-mg tablet per day on Tuesday, Wednesday, Thursday, Saturday and Sunday, and one and one-half tablets on Monday and Friday). Today her international normalized ratio (INR) is 3.6. Looking back at her records, you notice that her INR trend has gradually risen; her last value was 2.9. How should you adjust her warfarin dose?

What the evidence says

To provide safe, effective care for patients receiving warfarin, practices must follow a systematic process for managing anticoagulation and adjusting warfarin doses. Too often, however, the management of anticoagulation is haphazard because of busy clinical practices and multiple competing demands for physicians' time and attention.

Two alternatives to managing anticoagulation in the primary care office are using anticoagulation management services (AMS) and patient self-monitoring (PSM), which relies on home testing of the INR. A systematic review of the evidence found some support for AMS and PSM over usual care because of increased patient time in the therapeutic range and fewer bleeding complications.¹ However, most studies were small, and many were non-randomized. When patients can choose whether to attend an AMS or get usual care, it is possible that more motivated, compliant patients are attracted to the AMS. Large-scale randomized controlled trials with an appropriate duration of follow-up are lacking. Two trials using a before-and-after design (which is still more subject to bias than a prospective randomized trial) found more hemorrhages and recurrent thromboembolia in the usual care group than in the group managed by AMS.^2,3 However, the single prospective randomized trial on this topic followed 363 patients for two years and found no difference in complications or time spent in the therapeutic range.⁴

What distinguishes AMS and PSM from usual care is that they take a consistent, systematic, protocol-driven approach to monitoring patients. Put another way, they make it easy to do the right thing and hard to do the wrong thing when adjusting warfarin doses. By using standard protocols and making the best possible use of nurses and other staff, physicians should be able to improve outcomes to a similar extent in their own offices.

Unfortunately, no randomized trials exist comparing different dosage adjustment algorithms. The algorithm shown in the flow sheet is adapted from that of the anticoagulation service at the University of Michigan⁵ and is consistent with recommendations from the American College of Chest Physicians guideline¹ and others. A chart to help patients remember the correct dose of warfarin is also provided. To use it, simply identify the correct weekly warfarin dose, highlight that row in the table, and give the chart to the patient.

The answer

To treat the fictitious patient mentioned earlier, refer to the table at the bottom of the flow sheet on page 79. It suggests that you lower Mrs. Leiden's warfarin dose 5 to 10 percent (e.g., to 36 to 38 mg) and recheck her INR in 7 to 14 days.

If she returns in one week with a therapeutic INR, you would ask her to come back in one week. The next week, if she is in range again, you would ask her to come back in two weeks.