Diagnosing TTP. (A) Without immediate access to ADAMTS13 data, other criteria must be used to diagnose patients with microangiopathic hemolysis and thrombocytopenia who may have TTP (pink), STEC–HUS (yellow), aHUS (green), or secondary thrombotic microangiopathy (TMA; white). This approach misclassifies some patients. In addition, a few patients without microangiopathic hemolysis or thrombocytopenia will not be recognized promptly. Some of these errors can be corrected when ADAMTS13 results become available (solid arrows), and others become evident from the clinical course (dotted arrows). (B) Immediate access to ADAMTS13 data prevents the misclassification of patients with TTP, after excluding rare secondary causes of ADAMTS13 deficiency, such as liver failure. In addition, immediate testing facilitates the identification of patients with atypical presentations of ADAMTS13 deficiency, including the absence of microangiopathic hemolysis or thrombocytopenia.