Biological variation of International Normalized Ratio

for prothrombin times, and consequences in monitoring oral anticoagulant therapy: computer simulation of serial measurements with goal-setting for analytical quality Jens Kjeldsen,1 Jens Flensted Lassen,2,4 * Per Hyltoft Petersen,3 and Ivan Brandslund4 Oral anticoagulant therapy (OAT) has a well-estab-lished efficacy in prophylaxis and treatment of throm-boembolic disorders. Because complications are related to intensity of OAT, optimal control of treatment is mandatory. In studies of OAT, as many as 30 % of International Normalized Ratio (INR) measurements for prothrombin times fall outside the therapeutic interval. Preanalytical, analytical, and biological variation all contribute to this. Computer simulations of serial INR measurements were performed for various assume