 Inclusion Criteria: - Greater than or equal to 3 months history if ITP, regardless of splenectomy status, and completion of at least 1 prior treatment for ITP - 2 of 3 pretreatment platelet counts that were less than 30 x 109L if not currently on ITP therapy or less than 50 x 109L if currently receiving corticosteroids for ITP therapy - Ability to give informed consent Exclusion Criteria: - Known history of arterial thrombosis, active malignancy, or bone marrow stem cell disorder 