Cox proportional hazard regression analysis identifies the parameters associated with iTTP recurrence. HRs were determined in patients with low ADAMTS13 activity (<10 U/dL) 3 to 7 days after TPE (A), low ADAMTS13 activity (<10 U/dL) at clinical response/remission (B), low levels of ADAMTS13 antigen (the lowest quartile) at clinical response/remission (C), and high levels of anti-ADAMTS13 IgG (the highest quartile) 3 to 7 days after TPE (D) and at clinical response (E). Additional covariates were added to the analysis (as indicated in panels A-E). P values less than .05 and .01 are considered to be statistically significant and highly significant, respectively. WBC, white blood cells.