The invention generally relates to methods of testing the effectiveness of a von Willebrand factor (VWF) in treating von Willebrand disease (VWD) in a subject comprising measuring VWF cleavage fragments in a blood sample from the subject before and after treatment. In particular, the invention relates to methods of measuring VWF cleavage fragments, wherein an increase in VWF cleavage fragments after the treatment indicates that endogenous a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) in the subject is cleaving the VWF and wherein a decrease or absence of VWF cleavage fragments after the treatment indicates that endogenous ADAMTS13 in the subject is not cleaving the VWF.