A diagnosis of TTP is confirmed by a severe deficiency (<10%) of a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) activity before the first plasma exchange.1ADAMTS13 is the key regulator of the hemostatic activity of von Willebrand factor (VWF), accomplished by cleavage of a single site within the A2 domain of VWF.2Further assays in the diagnostic workup of immune-mediated TTP (iTTP) include identification of anti-ADAMTS13 immunoglobulin G (IgG) autoantibodies. This evidence concerns the gene ADAMTS13 and thrombotic thrombocytopenic purpura.