Determining von Willebrand factor activity comprises mixing a sample with a human GPIbalpha protein (I) having amino acid substitutions at positions 233 and 239 and not adding ristocetin or botrocetin. Independent claims are also included for: (1) test kit for an assay as above, comprising (I) on a nonparticulate solid phase; (2) producing (I) by expressing (I) recombinantly in a prokaryotic or eukaryotic cell culture and isolating (I) from a cell lysate or culture supernatant by affinity chromatography.
Process for the determination of the von Willebrand factor-cleaving activity of the ADAMTS-13 protease