vWFantigen

vWFAntigen, or von Willebrand factor antigen (vWF:Ag), is the concentration of von Willebrand factor in blood plasma as measured by immunoassay. The result is usually reported as a percentage of a reference standard (vWF:Ag %), or in international units per deciliter depending on the laboratory.

Von Willebrand factor is a large multimeric glycoprotein produced primarily by endothelial cells and megakaryocytes. It mediates platelet adhesion to subendothelial collagen and serves as a carrier for factor VIII, helping to maintain its plasma level. VWF exists in multimers of varying sizes, and the distribution of multimer sizes contributes to hemostatic function.

Clinical use: vWF:Ag is used together with tests of function to diagnose von Willebrand disease and to monitor therapy. A reduced vWF:Ag suggests quantitative deficiency; a normal or high vWF:Ag does not exclude a qualitative defect. The ratio of functional activity (for example, ristocetin cofactor activity, vWF:RCo) to antigen helps classify VWD types. Elevated vWF:Ag can occur in inflammatory states, pregnancy, smoking, obesity, and some malignancies; ABO blood group O tends to have lower baseline levels.

Limitations: vWF:Ag measures amount but not function. It can be influenced by age, sex, and acute illness, and values should be interpreted in the clinical context. In suspected VWD or bleeding history, a panel including vWF:Ag, vWF activity (vWF:RCo), and factor VIII is commonly ordered.