Improving Anticoagulation Measurement Novel Warfarin Composite Measure.

Background: Percent time in therapeutic range (TTR) and international normalized ratio (INR) variability both measure warfarin control and are associated with outcomes independently. Here, we examine the advantages of a warfarin composite measure (WCM), which summarizes the 2 when measuring patient outcomes. We also examine how the measure chosen would affect anticoagulation clinic performance rankings.
Methods and Results: We constructed WCM using an equally weighted method, adding standardized TTR to standardized log-transformed INR variability using 103 897 warfarin-experienced patients from 100 anticoagulation clinics. We examined the association of WCM with ischemic stroke, major bleeding, and fatal bleeding, using a subset of patients with atrial fibrillation (n=40 404). We divided patients into quintiles based on their level of control for TTR, log INR variability, and WCM. We calculated the hazard ratios for ischemic stroke, major bleeding, and fatal bleeding stratified by these quintiles. WCM hazard ratios for stroke and fatal bleeding showed the largest difference between excellent control and poorest control quintile compared with TTR and log INR variability, but not for major bleeding. In addition, we compared site rankings obtained using each of our 3 performance measures. Kappa scores for identifying outlier and nonoutlier clinics between WCM and its components were moderate (κ=0.56 for TTR and κ=0.62 for log INR variability) but was weak between TTR and log INR variability (κ=0.13).
Conclusions: WCM produces the largest range of risk for warfarin complications, widening the floor ceiling effects that limit the use of TTR and INR variability as separate measures. Anticoagulation clinics ranking changed considerably according to the anticoagulation measure that was selected.