Platelets are concentrated, washed and then lysed with a detergent. VWF in the supernatant is measured with a latex immunoassay which employs latex particles coated with antibodies to VWF to capture VWF in the supernatant. The VWF acts as a bridge between latex particles that leads to a degree of agglutination directly proportional to VWF concentration, which is measured turbidimetrically.
von Willebrand factor (VWF) is a large adhesive glycoprotein synthesised in endothelial cells and megakaryocytes. Unlike the activated coagulation factors of secondary haemostasis it is not an enzyme and its functions involve binding to cells and molecules. Upon vessel injury, VWF binds directly to exposed sub-endothelial collagen and remains anchored. Blood flow unravels anchored VWF to expose the binding site for the constitutively expressed platelet surface receptor glycoprotein Ib. VWF captures and tethers platelets arriving at the scene which promotes subsequent events of primary haemostasis towards formation of a platelet plug. VWF also serves as the plasma carrier of FVIII to protect it from proteolytic degradation and also to ‘deliver’ it to sites of injury and clot formation.
von Willebrand disease (VWD) is the most common hereditary bleeding disorder and the deficiency can be quantitative, involving reduced levels of normally functioning VWF, or qualitative, involving dysfunctional molecules. Some type 1 sub-types have reduced VWF in platelet α-granules and measurement can be useful. Measuring platelet VWF can be useful in detecting disorders of platelet α-granules.