Of the 14 IgG preparations in each APS and SLE/APS- group, 4 in APS and 7 in SLE/APS- displayed weak thrombin binding of less than 20% above the threshold for positivity compared with FXa binding which was between 40 to 60% above the relative cut-off for positivity (data not shown). Here, F10 is linked to systemic lupus erythematosus.