Avidity of IgG anti-factor Xa (FXa) antibodies in antiphospholipid syndrome (APS) and systemic lupus erythematosus (SLE)/APS-negative (APS-) groups. Percentage of maximum binding to FXa with NaCl at all concentrations tested. Bars represent mean ± standard error of the mean (SEM); **P <0.001, *P <0.05. Experiments were performed in duplicate for each sample. Results are representative of at least three independent experiments.