Managing warfarin therapy so drug levels are both safe and effective is an ongoing challenge for clinicians because not enough research has been done on warfarin management strategies. To define an optimal dosage management strategy for this important anticoagulant, researchers studied 3,961 patients taking warfarin from 94 community-based clinics and followed them for an average of about 15 months. They concluded that therapy could be improved "considerably" if clinicians change the warfarin dosage only when the patient's international normalized ratio is 1.7 or lower, or 3.3 or higher. The more time a patient spends outside this therapeutic range, the greater his risk of complications such as stroke and gastrointestinal hemorrhage.
Source: Rose AJ, Ozonoff A, Berlowitz DR, Henault LE, Hylek EM. Warfarin dose management affects INR control. J Thromb Haemost. 2009;7(1):94-101.