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vWF Quick Facts
- Molecular weight: 500 – 20 000 kD
- Synthesis: Endothelial cells, megakaryocytes
- Half-life: 6 – 12 hours
- Plasma concentration: 5 – 10mg/l
- Normal range: 50 – 150% 0.5 – 1.5E/ml (dependent of blood group)
Biochemistry of vWF
von Willebrand-factor (vWF) is part of the factor VIII molecule complex. The vWF is formed by subunits of 220 000 D that form dimers. Aggregates of 30-40 of these dimers form the multimer structure of the vWF-molecule. The vWF causes the adhesion of activated platelets to the subendothelium (damaged tissue wall) respectively the aggregation of platelets over the membrane receptors (Glycoprotein Ib and Glycoprotein IIb/IIIa), the so-called ristocetin-cofactor-function. Thus the vWF has an important function in the primary hemostasis (temporary closure of a tissue wall damage with platelet aggregates). As carrier molecule for non-covalent bound factor VIII the vWF prevents its proteolytic degradation and secures the transport to places where factor VIII is necessary as cofactor to the plasmatic coagulation (secondary hemostasis). Von Willebrand Factor is also known as Ristocetin-Cofactor (Factor VIII:RiCo) Factor VIII-associated antigen (Factor VIII:Ag).
Clinical significance of vWF
In most subjects the von Willebrand-syndrome disease (also called Willebrand-Jürgens-syndrome) leads to prolonged bleeding times. It is the most frequent congenital bleeding disorder and is distinguished as quantitative (Type I and III) and/or qualitative defects (Type II) in the vWF-molecule. The severity of the disease varies. Mild forms may be without symptoms. In severe cases surgery, tooth extractions and also menstruation cause a risk of bleeding to death. Increased vWF-levels are seen e.g. as consequence of acute phases, stress situations and liver diseases (values to 1000%) as well as post-operatively and post-traumatically.
Clinical or Research use of vWF
- Differential diagnosis between hemophilia A and von Willebrand-Syndrome
- Detection of acquired von Willebrand-Syndrome
- Additional diagnosis by liver- and vascular diseases
- Diagnosis of decreased proteolysis of vWF by Thrombotic-thrombocytopenic Purpura (TTP)
vWF Assay Measurement
The diagnosis of von Willebrand Disease (VWD), probably the most common congenital bleeding disorder, requires a number of special tests at the laboratory level. The measurement and comparison of von Willebrand Factor Antigen (VWF:Ag), VWF Activity and Factor VIII (FVIII) levels in plasma aid in the differentiation of quantitative defects (type 1 or type 3) or qualitative defect (type 2) of VWF and therefore to diagnose the different types of VWD.
When an extremely low or undetectable level of VWF:Ag is obtained, a type 3 VWD could be expected. If a moderate or even normal result is obtained, VWF Activity and FVIII assays must be performed and compared with the VWF:Ag result. If all three values are within the normal range, VWD and Hemophilia A may be excluded. If at least one parameter is abnormally low, it is necessary to calculate the ratios VWF Activity/VWF:Ag and FVIII/VWF:Ag. If both ratios are close to 1 (some authors suggest 0.7 as cut-off), a VWD type 1 may be diagnosed.
When the VWF/Activity/VWF:Ag ratio is low (0.7 is also the suggested cut-off), types 2A, 2B or 2M may be diagnosed. These subtypes are characterized by its abnormal multimeric pattern and/or its altered platelet affinity. Additional laboratory tests as RIPA (Ristocetin Induced Platelet Aggregation), multimeric analysis and binding assays are required in order to be able to distinguish the different subtypes. When the FVIII/VWF:Ag ratio is low (0.7 is also the suggested cut-off), a type 2N or Hemophilia A may be diagnosed and a FVIII binding assay is necessary to discriminate among them.
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