Frequency and significance of schistocytes in TTP/HUS patients at the discontinuation of plasma exchange therapy.

INTRODUCTION: Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS) is characterized by thrombocytopenia, a microangiopathic hemolytic anemia (presence of schistocytes) and elevated LDH without another likely explanation. Standard of care is daily plasma exchange, which is typically discontinued when the platelet count exceeds 100-150 x 10(9)/L for 2 days. However, residual schistocytosis, the presence of schistocytes at the time of discontinuation of plasma exchange therapy, is often disconcerting. We evaluated the frequency and significance of residual schistocytosis in TTP/HUS patients when the patients' platelet counts returned to normal levels (e.g., 100-150 x 10(9)/L). METHODS: Retrospective review in our institution from 01/1999-03/2004 of all patients treated with plasma exchange for TTP/HUS with at least 2 months of follow-up for relapse was completed. Patients were excluded if the clinical course was complicated by HIV, stem cell/bone marrow and solid organ transplant, pregnancy and auto-immune disease. Schistocytes were documented on day of presentation and on the day the platelet count reached 150 x 10(9)/L. Grading scale (using 100 x objective-a high power field, with approximately 100 red blood cells per field) for schistocytes was as follows: rare for 1 schistocyte per every other other field, 1+ for 1-5%, 2+ for 6-15%, and 3+ for >15%. The frequency of schistocytes was compared to frequency of relapse within 2 months, using Fisher's exact test. RESULTS: We identified 57 patients with TTP/HUS who received plasma exchange therapy. Of these patients, 12 did not have a follow-up microscopic examination of a peripheral blood smear at discontinuation of plasma exchange therapy and were excluded from further analysis. Of the remaining 45 patients, 16 had residual schistocytosis (35.6%). There was no statistically significant difference in relapse rate with or without residual schistocytosis (P = 1.00, Fisher's Exact test, 2 sided). CONCLUSIONS: In this study, we found that the presence of residual schistocytosis is common (35.6%). The presence of residual schistocytosis, however, was not predictive of relapse.