Behaviour of the Two-Step Coagulation Assay to Identify Antib2-glycoprotein I (ab2GPI)-Lupus Anticoagulant (LA) in Patients with Antiphospholipid Antibodies

Abstract number: P0615

Gennari LC, Blanco AN, Alberto MF, Grosso S, Lazzari MA

A two-step assay to distinguish anti-human b2GPI-LA from other antibodies with LA activity that employs two different final calcium concentrations (10 mM and 5 mM), has recently been published. We evaluate the behaviour of our routine dRVVT using both calcium concentrations, in LA (+) or (-) and in samples with (+) or without (-) IgG anticardiolipin (ACA). Results were expressed as ratios (R) between the coagulation time of either plasma sample or a 1 : 1 mixture of patient and control plasmas (P : N) divided by the control plasma clotting time, for each calcium concentration (R10, R5). Samples were divided in A: 11 LA-, B: 11 LA+/ACA-, C: 10 LA+/ACA+, D: 5 P : N LA-/ACA-, E: 5 P : N LA+/ACA-, F: 13 P : N LA+/ACA+. LA was defined according to the SSC-ISTH criteria, based on dRVVT (Sigma) and aPTT (PTT-LA, Stago); ACA were detected using a commercial kit (The Binding Site). No significant difference was found in the controls by decreasing calcium concentrations (AR10: 1.00 ± 0.03; AR5: 1.00 ± 0.02). In LA + groups, the ratios increased in relation to the controls; however, the differences between both calcium concentrations were not significant (BR10: 1.22 ± 0.06; BR5: 1.30 ± 0.07; CR10: 1.60 ± 0.17; CR5: 1.67 ± 0.22). Moreover, non-significant (P= 0.121) differences were observed between ACA- (B) or ACA+ (C) groups at 5 mM final calcium concentration. In mixing samples a significant (P < 0.05) decrease in the coagulation times was observed by decreasing calcium concentration (DR10: 0.99 ± 0.03, DR5: 0.81 ± 0.03; ER10: 1.09 ± 0.02, ER5: 0.93 ± 0.04; FR10: 1.32 ± 0.08, FR5: 1.15 ± 0.06); but without significant differences (P= 0.122) between ACA- (E) or ACA+ (F) groups at 5 mM final calcium concentration. Although the ACA's assay using in this study would be a b2GPI dependant assay, the differences observed in our data in comparison with those reported for positive ab2GPI samples, may suggest a different behaviour between ACA-b2GPI dependant and ab2GPI antibodies.