ADAMTS13 activity measurements by both immunoblotting and the fluorogenic assay with FRETS-VWF73 substrate. The size of the circle corresponds to number of observations. For example, in the lower left corner of panel A, the large circle represents 33 observations. (A) Measurements on 261 patients at the time of diagnosis of their first episode of TTP. The immunoblotting technique at higher ADAMTS13 activity is semiquantitative with activity measured at increments of 5%. (B) Measurements of one normal human plasma sample serially diluted into congenital TTP (Upshaw-Schulman syndrome) plasma. Ten replicates of 6 dilutions (0%, 5%, 10%, 20%, 40%, and 100%) each were measured on 10 different days in a blinded fashion. These data from our laboratory have been previously published as part of a multicenter collaborative study.²⁶