Plasma-derived human VWF (pVWF) was purified from commercial VWF/FVIII concentrate by gel filtration with a Sepharose 4B-CL column (Amersham Pharmacia Biotech AB, Uppsala, Sweden) and heparin-Sepharose 6FF (Pharmacia) affinity chromatography, as reported [22] . VWF antigen (VWF:Ag) concentration was determined by an enzyme-linked immunosorbent assay (ELISA) kit from Dako (Glostrup, Denmark) and VWF multimers analysis was performed as reported [23] .