 Measurement of ADAMTS-13 activity. ADAMTS-13 activity was measured in citrated plasma samples by a fluorescence resonance energy transfer FRET-based assay using a VWF86 amino-acid peptide substrate contained in a commercial kit from Instrumentation Laboratory ADAMTS-13 activity, Werfen Group, Milano, Italy and a Varian Eclipse spectrofluorometer, as previously detailed 10, 13 and expressed as of normal plasma. 