Detection of activated VWF in TTP plasma. (A,C) AU/VWFa-11–coated microtiter wells were incubated with NPP (▪), plasma from healthy individuals (n = 9; □), and plasma from patients suffering from acquired TTP (A; n = 12; ▪) or congenital TTP (C, n = 5, ▴). Plasma was diluted before incubation to obtain a concentration range of VWF (0.23-0.93 nM). Bound VWF was monitored with HRP-conjugated anti-VWF antibody. The amount of VWF in the diluted sample was plotted against the HRP activity (OD 490 nm). Slopes were calculated and the slope found for NPP was set to be 1. Arrows indicate the slopes found for a patient suffering from acquired TTP (1) or congenital TTP (2). (B-D) The activation factor was calculated and plotted in a scatter plot. Arrows indicate the values found for the patients plotted in panels A and C. Activation factors found for acquired and congenital TTP were significantly higher than those for the healthy individuals (acquired TTP versus healthy, P < .001 and congenital TTP versus healthy P < .03). The activation factor found for congenital TTP was also significantly elevated when compared to acquired TTP (P < .05). Data represent the mean ± SD.