A simple method to discriminate between β2-glycoprotein I- and prothrombin-dependent lupus anticoagulants

Lupus anticoagulants (LAC) are a heterogeneous group of autoantibodies that prolong phospholipid-dependent clotting assays. The autoantibodies that cause LAC activity are predominantly directed against beta(2)-glycoprotein I (beta(2)GPI) or prothrombin. In the present study, we describe a method to differentiate between LAC caused by antibodies directed against beta(2)GPI or prothrombin. Monoclonal antibodies, affinity purified patient antibodies, and selected patient samples were used to show that in an aPTT-based clotting assay (PTT-LA; Diagnostica Stago), the use of cardiolipin vesicles in the neutralization procedure discriminates between beta(2)GPI- or prothrombin-dependent LAC activities. Addition of cardiolipin vesicles shortened the prolonged clotting time caused by anti-beta(2)GPI antibodies with LAC activity, whereas this procedure further prolonged clotting times caused by antiprothrombin antibodies with LAC activity. In contrast, addition of phosphatidylcholine/phosphatidylserine vesicles corrected prolonged clotting times caused by either anti-beta(2)GPI or anti prothrombin antibodies with LAC activity. The effects of cardiolipin (CL) on beta(2)GPI-induced LAC activity were specific for contact activation mediated clotting assays. Possible explanations for these findings are the relatively high affinity Of beta(2)GPI for cardiolipin, as determined by surface plasmon resonance analysis, and inhibition by anti-beta(2)GPI antibodies of the CL-induced prolongation of the PTT-LA.