Increased amounts of active VWF in patients with anti–β₂ GPI antibodies. (A) Plasma of a healthy individual (■), a patient positive for lupus anticoagulant not due to anti–β₂ GPI antibodies (□), and a patient positive for lupus anticoagulant due to anti–β₂ GPI antibodies (n = 10, ●) were diluted to obtain similar amounts of VWF antigen. Indicated concentrations of VWF were added onto the AU/VWFa-11–coated wells. The initial linear part of the binding curve is shown. Plasma sample of a patient with acquired TTP is included as positive control (○). (B) The ratio of the slopes of the initial linear parts for the different plasma samples (n = 29 for controls and n = 10 for each patient group) over the slope for NPP was calculated and represents the relative amount of circulating active VWF. Relative amounts of active VWF are presented in a box-and-whiskers plot. (C) Patients were divided into those with (n = 11) or without (n = 9) thrombosis, and relative amounts of active VWF in these groups are presented in a box-and-whiskers plot.