von Willebrand Factor (vWF) is a plasma glycoprotein that circulates as disulfide linked multimers ranging in size from 520 kDa dimers to greater than 20,000 kDa. Each 260 kDa 2,050 amino acid monomer binds one Factor VIII molecule protecting against proteolytic degradation and clearance. vWF also binds platelets and collagen and is required for platelet plug formation. ADAMTS13 cleaves vWF at vascular injury sites, in circulation, and anchored on the endothelial surface. Type 1 von Willebrand disease due to quantitative vWF deficiency is characterized by mild bruising and bleeding. The sensitive quantitative measurement of total human vWF antigen (VWF:Ag) in plasma and serum samples is easily performed with this 96 well strip format ELISA kit. The concentration of vWF in normal human plasma is approximately 10.0 µg/ml. The normal range is 0.5-2 IU/ml with values less than 0.5 IU/ml indicating type 1 von Willebrand disease. The assay measures human vWF in the 0.5-500 ng/ml range. Samples giving human vWF levels above 500 ng/ml should be diluted in blocking buffer before use. A 1:200-1:1,000 dilution for plasma or serum is suggested for best results. Human vWF will bind to the monoclonal capture antibody coated on the microtiter plate. After appropriate washing steps, biotinylated monoclonal anti human vWF primary antibody binds to the captured protein. Excess primary antibody is washed away and bound antibody is reacted with peroxidase conjugated streptavidin. Following an additional washing step, TMB substrate is used for color development at 450nm. A standard calibration curve is prepared using dilutions of human vWF and is measured along with the test samples. Color development is proportional to the concentration of vWF in the samples. All reagents and standards are provided in these ELISA kits.