A new composite measure that incorporates both time in therapeutic range and international normalized ratio variability appears to be more accurate than either of these tests alone at monitoring warfarin anticoagulation, according to a report published online Sept. 29 in Circulation: Cardiovascular Quality and Outcomes.
Time in therapeutic range (TTR) and international normalized ratio (INR) variability track two different aspects of anticoagulation control. TTR measures the percentage of time that patients spend within the therapeutic range while taking warfarin, which reflects the amount of time that treatment intensity was appropriate. INR variability measures variations in the stability of warfarin’s anticoagulation effects over time. Most clinicians monitor their patients using one or the other of these measures, usually TTR. A composite measure that combines the two “would encourage providers to focus on all components of anticoagulation control, not just those measured by the current standard of care, TTR,” said Dr. Zayd Razouki of the Center for Health Services Research in Primary Care, Durham (N.C.) Veterans Affairs Medical Center, and his associates.
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It is important to note that a substantial number of patients taking warfarin could be classified as having poor control of anticoagulation by one of these measures, but good control by the other. Combining TTR with INR variability would ensure that both appropriate intensity and appropriate stability of warfarin therapy were being used to judge each patient’s anticoagulation control, they noted.
The investigators devised such a measure, a summary score they called WCM (warfarin composite measure). They then compared the performance of all three measures at predicting major warfarin-related complications, using as a sample population 40,404 participants in the Veterans Affairs Study to Improve Anticoagulation (VARIA). These study subjects were aged 65 years or older (mean age 76), had been taking warfarin for at least 6 months to treat atrial fibrillation, and were followed for an average of 14 months for the development of ischemic stroke, major bleeding, and fatal bleeding.
A total of 3.1% of these patients developed ischemic stroke, 6.4% developed major bleeding, and 0.9% developed fatal bleeding while taking warfarin. After the data were adjusted to account for numerous potential confounding factors, WCM correlated most closely with risk for adverse warfarin-related clinical events, Dr. Razouki and his associates wrote (Circ Cardiovasc Qual Outcomes. 2015 Sep 29. doi:10.1161/circoutcomes.115.001789).
Their findings also indicate that WCM may be a more accurate measure of a clinic’s or a medical system’s performance at monitoring anticoagulation than either their average TTR or INR variability alone.
The study results may not be generalizable to all populations, because this VA sample was overwhelmingly male (98%), the investigators added.