POTENTIAL FAILURE OF THE INTERNATIONAL NORMALIZED RATIO (INR) SYSTEM IN THE MONITORING OF ORAL ANTICOAGULATION IN PATIENTS WITH LUPUS ANTICOAGULANTS

Patients with antiphospholipid antibody syndrome (APS) experience a hi gher rate of recurrence of thrombosis than the general population of p atients with thrombotic disease. Based on a retrospective analysis, it has been suggested that patients with APS should be kept on prolonged anticoagulation aiming at international normalized ratio (INR) values > 3.0. To evaluate whether the requirement for more intense anticoagu lation depends on the variable sensitivity of thromboplastin reagents to the influence of aPLA, we monitored oral anticoagulant treatment in 10 patients with persistent lupus anticoagulants (LA) and venous thro mboembolic disease using two thromboplastin reagents : Pro-IL-Complex (Instrumentation Laboratory, combined) and Recombiplastin (Ortho, reco mbinant), A cenocoumarol dosage nas always assigned based on INR value s obtained with the combined thromboplastin using diluted (1:20) test plasma, aiming at an INR interval of 2.0 to 3.0. single INR determinat ions with both reagents were obtained throughout the study period for 110 aPLA-free patients on stable oral anticoagulation, Using the manuf acturer's instrument-certified international sensitivity index (15:1) values, INR obtained with the recombinant reagent were significantly h igher than those obtained with the combined reagent in LA-positive pat ients, but they vr-ere lower in LA-negative patients, After correction for local ISI calibration in LA-negative patients, INR values of 3.1 and 4.6 with Recombiplastin corresponded, respectively, to INR values of 2.0 and 3.0 with Pro-IL-Complex. These results indicate the thrombo plastin-dependency of INR values in patients with LA, thereby question ing tile validity of the INR system for the monitoring of oral anticoa gulant treatment in these patients.