Subcommittees on Control of Anticoagulation and Lupus Anticoagulant/Antiphospholipid Antibodies: Monitoring patients with the lupus anticoagulant while treated with vitamin K antagonists Patients on vitamin K antagonists (VKA) need frequent dose-adjustment based on the prothrombin time (PT) test, with results expressed as the international normalized ratio (INR). The need for the INR stems from the evidence that PT results obtained with commercial thromboplastins are varied owing to their different sensitivity to the coagulation defect induced by VKA. Consequently, PT results obtained for the same patient cannot be compared across laboratories. Furthermore, the ‘universal’ therapeutic interval stemming from clinical trials cannot be applied to individual patients. To minimize the between-thromboplastin differences the World Health Organization (WHO) endorsed a system of PT standardization whereby commercial thromboplastins are calibrated against an international standard to determine the international sensitivity index (ISI); that is, a measure of the sensitivity of any given commercial thromboplastin relative to the international standard. Continue reading.