Von Willebrand Factor Antigen

Von Willebrand Factor (vWF) is an important coagulation protein. It is responsible for platelet adhesion at high shear rates and acts as a carrier protein for Factor VIII. Low levels of vWF are found in von Willebrand's Disease which is the most common inherited blood clotting disorder, with a frequency of 1 - 3% in the caucasian population.

Von Willebrand Factor antigen in plasma is determined by immunological methods. Three types of immunoassay for vWF antigen are in general use.

Immunoelectrophoresis ('Rocket' electrophoresis) employs an agarose gel containing an antibody to vWF. The test plasma is placed in a well cut into the gel. During electrophoresis, the antigen-antibody complexes form a rocket-shaped precipitate, the height of which is proportional to vWF concentration.

Enzyme-linked immunoassay (ELISA) utilizes microtiter plates in which the plastic wells have been coated with an antibody to vWF to 'capture' vWF in the test plasma. The captured vWF is then detected with a second antibody conjugated to a chromophoric group. Thus, the optical density is proportional to the plasma vWF concentration.

Latex immunoassay (LIA) tests use latex particles that are coated with an antibody to vWF. The vWF in the test sample forms a 'bridge' between adjacent antibody-coated latex particles which then agglutinate. The rate at which agglutination occurs can be monitored by a change in optical density and this is proportional to the plasma vWF level..

The Hemostasis and Thrombosis Laboratory currently runs the LIA test system on an automated coagulation analyzer system in parallel with the vWF activity assay.

The normal range for vWF antigen in our laboratory is 55 - 160%. Values less than 40% are definitely abnormal and are indicative of von Willebrand's disease. It should be noted that subjects of blood group O tend to have lower vWF levels and this should be considered when interpreting borderline results.

This assay is an important test in the work-up for suspected von Willebrand Disease. However, complete evaluation of a patient requires determination of other factors including von Willebrand Factor Activity, von Willebrand Factor Multimers, Factor VIII activity, Platelet Function Analysis (PFA-100), blood group and family history