Proposed low VWF diagnostic algorithm. FVIII:C, FVIII clotting activity; VWF:CB, VWF collagen binding activity; VWF:GPIbR, Ristocetin-induced binding of VWF to a recombinant wild-type GPIb fragment; VWF:GP1bM, spontaneous binding of VWF to a gain-of-function mutant GPIbM fragment; VWF:RCo, VWF ristocetin cofactor activity.