Anti-factor Xa antibodies in patients with antiphospholipid syndrome and their effects upon coagulation assays.

Introduction: The aim of this study was to examine the prevalence and functional effects of antibodies directed against Factor (F)Xa and other serine proteases (SP) in patients with antiphospholipid syndrome (APS).

Methods: Serum from patients with APS (n=59), systemic lupus erythematosus (SLE; n=106), other autoimmune rheumatic disease (ARD; n=63) and 40 healthy controls (HC) were tested for IgG activity against thrombin (Thr), FXa, FVIIa, phosphatidylserine (PS)/FXa and antithrombin (AT)-III by enzyme-linked immunosorbent assay (ELISA). Anti-FXa positive IgG were purified to measure their avidity by chaotropic ELISA and functional effects upon clotting time (FXa-ACT) and FXa enzymatic activity (±AT-III).