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| Test Code: |
VWAG |
| Description: |
Von Willebrand disease is a bleeding disorder characterized by decreased or abnormal von Willebrand factor (vWF). vWF is a glycoprotein synthesized by endothelial cells and megakaryocytes. The molecule is assembled into high molecular weight multimers and stored in intracellular organelles of platelets and endothelial cells. Following stimulation by agonists such as DDAVP, vWF is secreted into the blood. The multimer size is regulated by a vWF protease on the vascular endothelium. vWF serves two primary roles in coagulation. First, vWF is a bridging molecule linking platelet surface adhesion molecules to subendothelial matrix proteins, and is responsible for platelet adherence to subendothelium, which occurs with endothelial injury. Secondly vWF also has a major biological function to stabilize factor VIII. Thus, the basis for the screening tests is that decreased or abnormal vWF results in defective platelet adherence manifested by prolonged bleeding time and decreased ristocetin-induced platelet aggregation (ristocetin co-factor activity. If vWf is decreased, factor VIII levels also decrease and may be reflected by a prolonged PTT. There are three general categories of von Willebrand disease (vWD): Type 1 disease is most common and characterized by a decrease in antigenic and functional vWF. Patients typically have a history of mucosal or skin bleeding, menorrhagia or prolonged bleeding following dental extractions. Type II vWD is characterized by an abnormal vWF molecule resulting in decreases in high molecular weight multimers. Types I and II are inherited in an autosomal dominant pattern. Distinction between type I and II disease requires vWF multimer analysis. Within type II vWD, differentiation between subtypes IIA and IIB requires ristocetin-induced platelet aggregation studies. Type III vWD is an autosomal recessive condition with low or undetectable vWF and low factor VIII, with bleeding complications comparable to hemophilia. |
| Indications: |
Suspected von Willebrand disease. It is important to note that vWF is an acute phase reactant and that levels fluctuate. Therefore, a patient with a bleeding history compatible with mild vWD may require testing on multiple occasions to exclude this diagnosis. Patients with type O blood normally have lower levels of vWF and factor VIII. |
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