The highest rate of mortality was seen in patients with a high anti-ADAMTS13 antibody level and low ADAMTS13 antigen level. A total of 72.7% of these patients was found to have strong inhibitory antibodies on 50:50 mixing studies. The lowest rate of mortality was seen in patients with an antibody level in the lowest quartile (LQ); only 7% of these patients were found to have strong inhibitory antibodies. ADAMTS13 antigen levels plotted against anti-ADAMTS13 IgG antibody levels. Results in the upper or lower quartile for either factor were tested to confirm the presence of strong inhibitory antibodies (defined as those in which ADAMTS13 activity remained <10% after 50:50 mixing studies). These were most frequently seen in those with a low ADAMTS13 antigen level and a high anti-ADAMTS13 IgG antibody level. The largest cluster can be seen in those samples that fell into both of these groups with a corresponding increased level of mortality.