VWF and thrombotic thrombocytopenic purpura: To confirm the diagnosis of iTTP, plasma ADAMTS13 activity and inhibitors or anti-ADAMTS13 IgGs must be performed in a timely manner.18ADAMTS13 is a plasma metalloprotease that cleaves von Willebrand factor (VWF), thus regulating normal hemostasis.19, 20, 21If plasma ADAMTS13 is less than 10 U/dL (or 10% of normal), TTP diagnosis is confirmed; if plasma ADAMTS13 is greater than 20 U/dL (or 20% of normal), the TTP is essentially ruled out.