Interference of lupus anticoagulants in prothrombin time assays: implications for selection of adequate methods to optimize the management of thrombosis in antiphospholipid-antibody syndrome

Background and Objectives. Prolonged anticoagulation aiming at Internationa l Normalized Ratio (INR) values >3.0 has been recommended for patients with thrombosis and the antiphospholipid-antibody syndrome. We evaluated the in fluence of anticoagulant antibodies in two different prothrombin time (PT) assays carried out on plasma from lupus anticoagulant patients on oral anti coagulation. Design and Methods. INR values obtained with a combined (final test plasma dilution 1:20) and a recombinant (final test plasma dilution 1:3) thrombopl astin were compared In 17 patients with persistent lupus anticoagulants (LA ) receiving oral anticoagulant treatment and monitored for 69.8 patient-yea rs. Doses of anticoagulant drugs were always assigned based on the results obtained with the combined thromboplastin, aiming at a target INR of 2.5 or 3.0 for patients with venous or arterial thromboembolic disease. Paired de terminations with both reagents were also obtained throughout the study per iod in 150 patients on stable oral anticoagulation but free of antiphosphol ipid antibodies. Total IgG fractions were purified from selected patients t o evaluate effect in the two PT assay systems. Results. No patient experienced recurrence of thrombosis or major bleeding complications (95% confidence interval: 0.1-6.5 pet 100 patient-years). INR values with the recombinant reagent were significantly higher than with th e combined reagent in 8 LA patients (mean Delta INR ranging from 0.17 to 0. 54) of the degree of anticoagulation was overestimated in all but one LA pa tients with the recombinant reagent when compared to the Delta INR observed in non-LA patients (-0.64 +/- 0.42). The anticardiolipin IgG titer (r(2) = 0.43, p = 0.004) and the anti-beta(2)GPI IgG titer(r(2) = 0.30, p = 0.023) were positively associated with the mean Delta INR observed in LA patients . When added to plasmas with different levels of vitamin K-dependent factor s, total IgG fractions from 6 LA patients with significant overestimation o f the INR with the recombinant reagent (mean DINR ranging from 0.17 to 0.54 , group 1) and from 7 LA patients with mean Delta INR less than or equal to 0.0 (ranging from -0.25 to 0.04, group 2) reproduced the effects observed ex vivo in the two assay systems. However, when total IgG fractions were te sted at the same final concentration in the two PT assay systems, there was no difference in the clotting times determined with total IgG fractions fr om group 1 and group 2 LA patients. Addition of negatively charged liposome s (0.4 and 0.8 mg/mL final concentrations) to platelet free plasma from LA- free patients on stable oral anticoagulation caused a 20% to 48% prolongati on of the prothrombin time determined with the recombinant reagent. In cont rast, no significant prolongation of the prothrombin time determined with t he recombinant reagent was observed upon addition of negatively charged lip osomes to plasma from group 1 LA patients. Interpretation and Conclusions. These results confirm previous suggestions of assay-dependency of INR values in LA patients on oral anticoagulation. F or these patients, accurate INR values may be obtained using combined throm boplastin reagents that permit testing at high plasma dilution. (C) 1999, F errata Storti Foundation.