Open ADAMTS13 conformation is a novel and early biomarker of subclinical iTTP. ADAMTS13 activity (blue squares), anti-ADAMTS13 autoantibody levels (red squares), and ADAMTS13 conformation (open circles, open ADAMTS13; filled light green circles, closed ADAMTS13) were determined in 39 consecutive plasma samples from 1 iTTP patient. Dates of sampling are indicated. The red arrows represent rituximab infusion. The vertical gray boxes highlight that ADAMTS13 conformation is open before a substantial decrease in ADAMTS13 activity (<10%). The blue dotted lines indicate 10% and 50% ADAMTS13 activity, while the red dotted line represents the cutoff for anti-ADAMTS13 autoantibody positivity (6.7%).