A 2016 guideline for practical management of warfarin therapy for treatment of venous thromboembolism (VTE) recommends INR testing should not exceed every six weeks for the first three months of warfarin therapy. After three months of therapy in patients with consistently stable INR, testing can increase up to 12 weeks. For INRs > 4.0 or <1.5, the Veterans Administration health care system suggests testing within seven days, and within 14 days for INRs 3.1 to 3.9 or 1.6 to 1.9. INR testing should not exceed three days following an INR > 5.0. If Vitamin K antidote was administered, next-day follow-up testing is recommended to avoid overcorrection of INR. [1]
According to the American College of Chest Physicians, patients with consistently stable INRs can be tested every 12 weeks rather than every four weeks. While there are no specific recommendations on the frequency of INR testing when initiating vitamin K antagonist (VKA) therapy, the guidelines state warfarin should be started with 10 mg for the first two days and then titrated based on INR measurements rather than the estimated dose. [2]
A 2015 review states that when initiating warfarin, INR should be measured daily for the first five days. Once the patient has two consecutive INRs in the target range, the INR can be tested at increasing intervals depending on its stability. When the warfarin dose and INR are stable, patients can usually be well controlled with INR measurements every four to six weeks, but some patients might require more frequent testing. Dose adjustment is not required for minor INR fluctuations if the result remains within the patient’s target range. [3]