Monitoring Oral Anticoagulant Therapy with Vitamin K Antagonists

Coumarin drugs are widely used to treat and prevent venous thromboembolism, to prevent stroke and systemic embolism in atrial fibrillation and in patients with prosthetic heart valves. Due to its relatively narrow therapeutic window, interaction with diet and other drugs, this treatment requires frequent dose-adjustment by means of laboratory testing. The test of choice is the prothrombin time (PT), because the large between-thromboplastin results variability needs to be standardized. The international normalized ratio (INR) is the current system of standardization endorsed by the World Health Organization (WHO). This chapter is aimed at discussing the principle of this system of standardization with particular emphasis to the thromboplastin calibration, including the WHO model and the establishment and hierarchy of international standards for thromboplastins; the local calibration; how the PT is converted into INR and the issue that may affect results; the validity limitations of the INR and the its implementation. This edition first published 2013