ADAMTS13antigen andconformation in the Japanese iTTP cohort. ADAMTS13 antigen (triangle, n = 81) was determined using ELISA by capturing plasma ADAMTS13 with anti-ADAMTS13 mAb 3H9 and detecting captured ADAMTS13 with biotinylated anti-ADAMTS13 mAbs 19H4 and 17G2. ADAMTS13 conformation (open conformation, open circle, and closed conformation, closed circle; n = 76) was determined using ELISA where open ADAMTS13 was captured by anti-ADAMTS13 mAb 1C4 and bound ADAMTS13 detected with biotinylated anti-ADAMTS13 mAb 3H9. mAb1C4 does not capture closed ADAMTS13. OD492nm values corrected for ADAMTS13 Ag levels resulted in the CI. Only samples with ADAMTS13 antigen levels >0.02 μg/mL could be used to determine the ADAMTS13 conformation as shown in right panel (n = 76). The detection limit of the ADAMTS13 antigen ELISA is 0.02 μg/mL (dotted line) and the cutoff for determination of open-closed ADAMTS13 is a CI of 0.5 with CI < 0.5 closed ADAMTS13 and CI > 0.5 open ADAMTS13 (dotted line). CI, conformation index; mAb, monoclonal antibody.