Patients with antiphospholipid antibody syndrome (APS) experience a higher rate of recurrence of thrombosis than the general population of patients 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 normalised ratio (INR) values > 3.0. To evaluate whether the requirement for more intense anticoagulation 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 thromboembolic disease using two thromboplastin reagents: Pro-IL-Complex (Instrumentation Laboratory, combined) and Recombiplastin (Ortho, recombinant). Acenocoumarol dosage was always assigned based on INR values obtained with the combined thromboplastin using diluted (1:20) test plasma, aiming at an INR interval of 2.0 to 3.0. Single INR determinations with both reagents were obtained throughout the study period for 110 aPLA-free patients on stable oral anticoagulation. Using the manufacturer's instrument-certified international sensitivity index (ISI) values, INR obtained with the recombinant reagent were significantly higher than those obtained with the combined reagent in LA-positive patients, but they were 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 thromboplastin-dependency of INR values in patients with LA, thereby questioning the validity of the INR system for the monitoring of oral anticoagulant treatment in these patients.