Factor (F) Xa reactive IgG isolated from patients with antiphospholipid syndrome (APS) display higher avidity binding to FXa with greater coagulant effects compared to systemic lupus erythematosus (SLE) non APS IgG. FXa signalling via activation of protease-activated receptors (PAR) leads to increased intracellular calcium (Ca2+). Therefore, we measured alterations in Ca2+ levels in human umbilical vein endothelial cells (HUVEC) following FXa-mediated PAR activation and investigated whether FXa reactive IgG from patients with APS or SLE/APS-alter these responses. We observed concentration-dependent induction of Ca2+ release by FXa that was potentiated by APS-IgG and SLE/APS-IgG compared to healthy control subjects' IgG, and FXa alone. APS-IgG and SLE/APS-IgG increased FXa mediated NF kappa B signalling and this effect was fully-retained in the affinity purified anti-FXa IgG subfraction. Antagonism of PAR-1 and PAR-2 reduced FXa-induced Ca2+ release. Treatment with a specific FXa inhibitor, hydroxychloroquine or fluvastatin significantly reduced FXa-induced and IgG-potentiated Ca2+ release. In conclusion, PAR-1 and PAR-2 are involved in FXa-mediated intracellular Ca2+ release in HUVEC and FXa reactive IgG from patients with APS and/or SLE potentiate this effect. Further work is required to explore the potential use of IgG FXa reactivity as a novel biomarker to stratify treatment with FXa inhibitors in these patients.