Detection of ADAMTS13 inhibitory autoantibodies by immunoblotting of VWF substrate degraded by BaCl₂-activated ADAMTS13 in mixtures of normal human plasma (NHP) with patient plasma obtained at acute initial TTP, or IgG purified from patient plasma (final NHP dilution, 1:20). Lanes 1-4: assay calibration by NHP dilutions of 1:20 (100% activity), 1:40 (50%, equivalent to an inhibitor titer of 1 BU/mL), 1:80 (25%), and buffer control (0%). Sister 1: mixtures of NHP 1:1 (vol/vol) with patient plasma (lane 5), with IgG purified from patient plasma (lane 6), and with IgG-depleted patient plasma (lane 7). Sister 2: mixtures of NHP 1:1 (vol/vol) with patient plasma (lane 8), with IgG purified from patient plasma (lane 9), and with IgG-depleted patient plasma (lane 10).