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Anticoagulants, such as warfarin, decrease the incidence of stroke in patients with atrial fibrillation (AF). The international normalized ratio (INR) is a measurement of the clotting ability of an individual’s blood plasma, and is used to guide anticoagulation in patients undergoing anticoagulation therapy. The therapeutic INR range for preventing embolic stroke in AF is 2.0-3.0. The time in therapeutic range (TTR) varies immensely based on individual patient characteristics, specific center operations, and country INR control methods. Apixaban is an alternative anticoagulant that is comparable in efficacy to warfarin and does not require INR monitoring. The Apixaban for Reduction in Stroke and Other Thromboembolic Events in Atrial Fibrillation (ARISTOTLE) trial was a recent study comparing the effectiveness of apixaban compared to warfarin in reducing incidence of stroke and systemic embolism. The authors studied whether variability in TTR across centers and individual patients altered the additional benefit seen with apixaban in comparision with warfarin.
The ARISTOTLE trial was double blind, double dummy study in which 18,201 subjects were randomized to receive either apixaban or warfarin. The enrolled subjects had AF and at least one other risk factor for stroke (TIA, heart failure, diabetes mellitus, hypertension, ?75 years of age, etc). Primary outcomes of the study were stroke, systemic embolism and the secondary outcome was all cause mortality. Primary safety outcome was major bleeding. Using a complex statistical model which incorporated variability in INR due to center and patient characteristics, estimation of center specific and predicted individual TTRs was performed. This was then utilized to compare the efficacy of Apixaban over warfarin across these center specific and individual predicted TTRs.
The median TTR (centre specific) in the warfarin arm was 66%. The incidence of stroke and systemic embolism was significantly lower in apixaban arm (1.27% per year) compared to warfarin group (1.60% per year) (HR= 0.79 [95% CI 0.66 -0.95]). The total mortality in apixaban group (3.52% per year) was lower than that in warfarin arm(3.94% per year)(HR = 0.80 [95% CI 0.80 -0.99] . Lastly there was decreased incidence of major bleeding with apixaban arm (2.13% per year) compared to coumadin group(3.09% per year)(HR 0.69[95%CI 0.60 – 0.80]. There was no difference in incidence of stroke, systemic embolism, total mortality or major bleeding when individual predicted TTRs were used to compare apixaban and warfarin.
In conclusion, this complementary statistical model demonstrated that benefit of apixaban over warfarin was present over a wide range of quality control with warfarin administration. Thus irrespective of center and patient specific INR control, apixaban was more effective than warfarin in preventing stroke, reducing mortality and decreasing significant bleeding. With the significant variation in INR quality and control across different centers, apixaban may provide a safer and more normalized method for anticoagulation therapy.
Reference: 1. (1) Wallentin, Lars, et al. "Efficacy and Safety of Apixaban Compared With Warfarin at Different Levels of Predicted International Normalized Ratio Control for Stroke Prevention in Atrial Fibrillation." Circulation 127.22 (2013): 2166-2176.
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