Selection of patients with iTTP for the study. A total of 164 patients with suspected TTP between April 2006 and June 2019 were referred for TPE. Of those, 67 patients had alternative diagnoses, 1 patient had hereditary TTP, 11 patients died during the acute episode, and 2 patients did not respond to treatment (1 later died, and 1 was lost to follow-up). These patients were excluded from the study. This left 83 unique patients (97 episodes) for analysis. Of these, 46 patients with iTTP had serial blood samples (≥3) and 48 patients had at least 2 samples collected during the treatment.