If undetected, acquired immune thrombotic thrombocytopenic purpura (iTTP) is an uncommon illness with a dismal prognosis. It is caused by the formation of autoantibodies against the von Willebrand factor (VWF) cleaving protease, A disintegrin, and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13). Caplacizumab, an immunoglobulin directed to the platelet glycoprotein Ibα receptor of VWF, has been shown to reduce iTTP faster than placebo. The laboratory assessment of VWF activity was dramatically lowered in caplacizumab clinical trials. Several VWF tests are available in the UK, and researchers for the study looked to see if there were any variations in VWF characteristics in 11 patients with iTTP who were receiving daily caplacizumab. Prior to caplacizumab medication, chromosomal factor VIII activity, VWF antigen, collagen-binding activity, VWF multimers, and six VWF activity tests were assessed, as were multiple times during treatment. 

All patients had normal or borderline normal VWF antigen and collagen-binding activity levels. Following therapy, all patients had ultra-large molecular weight multimers in their blood. VWF activity tests were normal or decreased with therapy, depending on the reagent and the patient. In the rare case of a caplacizumab-treated patient who required VWF activity assessment, laboratories must understand how their local reagents work since findings cannot be expected.