78 Thus, TTR provides a clearer picture of the quality of anticoagulation, guiding the clinicians to take appropriate steps to achieve better TTRs to balance the benefit–risk ratios.īefore initiating VKA therapy, the baseline INR should be determined and initial dose of VKAs should be administered. Even a good level of TTR, such as nearly 75% in the Netherlands, suggests that INR values are either too low or too high in 25% of the time while anticoagulant therapy is continued.įor achieving appropriate laboratory control, patients need to have their blood withdrawn about 20 times a year, which poses a significant burden. A country-wise analysis of Randomized Evaluation of Long-Term Anticoagulation Therapy (RELY) study data revealed that average TTR, excluding first week of treatment, varied from 44% in Taiwan to 77% in Sweden. Wide variations in TTR have been observed. The time within the therapeutic range (TTR) is a good overall measure of the quality of antithrombotic treatment with VKAs. Consequently, the reduction of factors X and II contributes to prolongation of the PT. Thus, it would be safe to state that during the first few days of VKA therapy, the PT reflects mainly a reduction of factor VII during the half-life (approx. The PT signifies the reduction of three pro-coagulant clotting factors (i.e., II, VII, X) out of the four vitamin K-dependent clotting factors that are reduced by warfarin at a rate proportional to their respective half-lives. The prothrombin time (PT) test is the most common test used to monitor VKA therapy. With the help of routine monitoring, such dangerous situations can be detected well in time allowing dose adjustment, as well as actions taken to prevent recurrence of such situations. Thus, the primary objectives of monitoring VKA therapy are to help in deciding the initial VKA dose and the maintenance doses on the basis of level of anticoagulation.Īpart from assisting in deciding the appropriate dosage regimen, monitoring helps in avoiding overcoagulation. Wide range of inter- and intra-patient variations in the dose requirement of VKAs that warrants the need of individualized dosage regimens to the desired international normalized ratio (INR) level. A fixed low-dose regimen of VKAs is not very beneficial. Many factors interfere with VKA uptake and metabolism by the liver, including food, other medications and comorbidities.
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