Platelet-dependent VWF activity assays. (A) VWF:RCo includes “traditional” assays that use ristocetin to induce VWF binding to platelets. (B) VWF:GPIbR includes assays based on ristocetin-induced binding of VWF to recombinant wild-type GPIbα fragments tethered to either latex particles or an enzyme-linked immunosorbent assay (ELISA) plate. (C) VWF:GPIbM assays are based on spontaneous binding of VWF to recombinant gain-of-function GPIbα fragments tethered to either latex particles or an ELISA plate