Inclusion Criteria:  Willing and able to provide written informed consent Diagnosis of ITP in adult patients (≥ 18 years of age) with an insufficient response to steroids ± immunoglobulins and requiring a change in therapy For Cohort 1, intent to initiate fostamatinib at enrollment as change in therapy For Cohort 2, currently undergoing treatment with fostamatinib as second line therapy for at least 12 weeks, with the intent to continue, at the time of enrollment  Exclusion Criteria:  Any prior ITP therapy/treatment other than steroids ± immunoglobulins (e.g. rituximab, thrombopoietin receptor agonists (TPO), splenectomy) Participation in any interventional study during the course of this study Lack of historical platelet counts, at time of fostamatinib initiation for those entering Cohort 2 Pregnant and/or nursing