AbstractThrombotic microangiopathy (TMA) syndromes are a heterogeneous group of microvascular syndromes that are typically treated with plasma exchange and other adjunctive therapies. Important pathogenic factors, such as ADAMTS13 deficiency, define distinct subsets of TMA. New treatments for TMA are being explored that are hypothesized to bring about remission more quickly. However, the existing factors that influence response to treatment time are poorly understood. We hypothesized that common laboratory parameters available at the time of treatment initiation might correlate with the number of days of plasma exchange required to induce remission. We therefore retrospectively compared pretreatment platelet counts, hematocrit levels, and C‐reactive protein (CRP) levels to the number of days of plasma exchange treatment in 27 ADAMTS13‐deficient TMA patients and 25 non‐ADAMTS13‐deficient patients that achieved remission. Using quantile regression analysis, we observed that in ADAMTS13‐deficient patients, higher initial hematocrit levels significantly correlated with shorter treatment response times. In addition, for ADAMTS13‐deficient patients, elevated levels of CRP correlated directly with longer response times. Higher platelet counts were associated with a nonsignificant trend toward shorter response times. In non‐ADAMTS13‐deficient TMA patients no significant correlations were observed. Our results suggest that when conducting clinical trials of adjunctive treatments for TMA, clinical data, including ADAMTS13 levels, pretreatment hematocrit levels, and CRP levels may be informative in interpreting response to treatment times. J. Clin. Apheresis, 2011. © 2011 Wiley‐Liss, Inc.