Elevated plasma DNA and MPO levels together with ADAMTS13 deficiency characterize the acute disease state in patients with acquired TTP. Analysis of 100 samples of patients with acquired TTP from the investigational cohort of which DNA, MPO, platelet counts, and ADAMTS13 activity were available. Comparison of ADAMTS13 activity to platelet count and plasma DNA (A) or MPO (B). Plasma DNA and MPO were elevated predominantly in ADAMTS13-deficient patients (< 10%) with thrombocytopenia (< 50 000 platelets/μL). (C) Correlation of plasma DNA and MPO with platelet counts. Both plasma DNA and MPO negatively correlated with the number of circulating platelets (*P < .05, **P < .01, ***P < .001).