Reduced von Willebrand factor activity UMLS:C4024701 HP:0008330 Von Willebrand factor (VWF) is a key hemostatic protein, but documenting its function through laboratory tests is not always straightforward. VWF serves as a carrier protein for factor VIII (FVIII), and also facilitates platelet adhesion through its interaction with platelet GPIb on the platelet surface and through its binding to the subendothelial matrix. This interaction is driven in vivo by shear stress, which induces a conformational change in VWF that allows it to bind platelet GPIb. In vitro, however, this interaction is induced by the antibiotic ristocetin, which enables VWF and platelet GPIb to interact in the absence of shear forces. Laboratory testing of VWF utilizes ristocetin in the VWF ristocetin cofactor activity assay (VWF:RCo), which is a measure of VWF binding to platelets, or by ristocetin-induced platelet aggregation (RIPA). Decreased activity of von Willebrand factor. Von Willebrand factor mediates the adhesion of platelets to the collagen exposed on endothelial cell surfaces. Decreased von willebrand factor activity Abnormality of von Willebrand factor