Low Dose Rituximab in Thrombotic Thrombocytopenic Purpura

Inclusion Criteria: 1. Age 18 or greater 2. Diagnosis of suspected thrombotic thrombocytopenic purpura (TTP) 1. Platelet count of \< 80,000 for newly diagnosed patients and \< 120,000 for relapsed patients 2. Microangiopathic hemolytic anemia with RBC fragmentation 3. LDH \>1 x ULN 3. Subjects who will receive treatment for TTP with plasma exchange 4. Subjects who have not started the 5th plasma exchange 5. Plasma ADAMTS13 activity \<10% Exclusion Criteria: 1. Treatment for TTP within the past 2 months 2. Severe active infection indicated by sepsis (requirement for pressors with or without positive blood cultures) or clinical evidence of enteric infection with E. coli O157:H7 or related organism 3. Currently under treatment for cancer (subjects with localized skin carcinoma will be accepted) 4. Microangiopathic hemolytic anemia due to a mechanical heart valve 5. Severe hypertension, as defined by systolic BP \>180 AND diastolic BP \>120, or papilledema 6. Organ or stem cell transplant 7. Use of calcineurin inhibitors (sirolimus, tacrolimus, cyclosporin A) within 6 months prior to diagnosis of TTP 8. Disseminated intravascular coagulation as defined by: a. INR \>2.0 (unrelated to anticoagulation, unresponsive to Vitamin K) or b. Fibrinogen \<100 mg/dl 9. Pregnancy 10. Known congenital TTP. 11. Rituximab within the previous year. 12. HIV history or positive serology 13. History of hepatitis B or positive serology for HBsAg or Anti-HBc 14. Persistent or unexplained platelet count below 150,000/μL within 3 months of current TTP presentation 15. Hypersensitivities or allergies to murine and/or humanized antibodies 16. Current participation in trials of investigational therapies or devices, other than central catheters