von Willebrand factor FVIII binding (VWF:FVIIIB)

Description: 
Type 2N VWD is characterised by markedly decreased binding affinity of VWF for FVIIII. There is a disproportional decrease in FVIII relative to VWF:Ag.

In the VWF:FVIII binding assay, patient VWF is captured by a VWF antibody bound to microtitre plate wells. Any FVIII that is bound to the VWF is removed with a high concentration of calcium chloride and exogenous recombinant FVIII is added to standardise the amount attempting to bind to test VWF. The VWF will capture FVIII relative to its FVIII binding capacity, the amount of binding being determined by addition of an enzyme-linked FVIII antibody that is reacted with a chromogenic substrate. The intensity of the coloured product of that reaction is directly proportional to the amount of FVIII bound to VWF.
Clinical details: 
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. Laboratory investigation of VWD encompasses a battery of assays that assess different aspects of the molecule which inform sub-classification and clinical management:

VWF:RCo assay measures glycoprotein Ib binding
VWF:Ag assay measures total protein concentration irrespective of function
VWF:CB assay measures collagen binding
VWF:FVIIIB assay measures FVIII binding
Multimer analysis investigates VWF structure
FVIII activity is measured as levels can be reduced due to reduction of its carrier.
Reference range: 

Interpretive reporting

Sample type and Volume required: 
External requests: Citrated platelet poor plasma
500µL x 1 aliquot
Internal requests: please refer to EPR label

Turnaround time: 
Contact laboratory
Special sample instructions: 

The sample should be analysed or manipulated & stored in the laboratory within 4 hours of venepuncture. Please ensure sample tubes are filled exactly to the fill-line as underfilling creates a dilution error and leads to inaccurate results.

Contacts:
Diagnostic Haemostasis and Thrombosis Department
020 7188 2797
St Thomas' Hospital
North Wing - 4th and 5th Floors
Westminster Bridge Road
London SE1 7EH

Laboratory opening times
24/7
For clinical advice or interpretation of results, please contact the laboratory in the first instance.

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Last updated: 09/03/2017