Von Willebrand Factor (vWF) is a glycoprotein that serves as a “glue” between subendothelial collagen and platelets. More specifically: subendothelial collagen ↔ vWF’s A3 domain ↔ vWF’s A1 domain ↔ GPIb platelet receptor. vWF also stabilizes factor VIII in circulation. vWF is composed of subunits that link together (dimerize) and subsequently combine to form functional multimers, most of which are in circulation.
Remember, hemostasis is a constant balance between clotting and bleeding. ADAMTS13 is a metalloprotease enzyme that limits vWF’s procoagulant effects by cleaving large multimers. In situations where this enzyme is defective, like thrombotic thrombocytopenic purpura, blood clots can form in vessels throughout the body leading to hemolysis, kidney injury, etc.
The severity of hereditary vWF disease (types I, II, and III) depends on the mutation; treatment is centered around the patient’s symptoms. An example of acquired vWF deficiency I see all the time as a cardiothoracic anesthesiologist and intensivist is due to shear stresses from aortic stenosis or mechanical circulatory support (e.g. ECMO or Impella). These can lead to conformational changes in vWF and early degradation by ADAMTS13.
As far as testing, vWF antigen testing determines “how many molecules” there are by looking for a subunit sequence. The ristocetin assay uses the antibiotic ristocetin to promote binding between vWF and the GPIb platelet receptor whereas the collagen-binding assay determines how vWF is binding to collagen.