Clinical Laboratory
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Florida Hospital Center for Thrombosis Research
2566 Lee Road
Winter Park, FL 32789

Phone: (407) 303-2440 (main office)
Phone: (407) 303-2449 (clinical lab)
Fax: (407) 303-2441

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Von Willebrand Factor Activity

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 (also known as Ristocetin Cofactor) activity in plasma is determined
by the agglutination of a standardized platelet suspension in the presence
of the antibiotic, Ristocetin. The Ristocetin Cofactor Activity is the in
vitro
activity of the von Willebrand Factor which is responsible for the
agglutination of platelets in the presence of Ristocetin. The ability of the
test plasma to support Ristocetin-induced platelet agglutination is compared
to that of dilutions of standard control plasma. The Hemostasis and Thrombosis
Laboratory performs this assay on an automated analyzer system that significantly
improves the precision of the technique.

The normal
range for vWF activity in our laboratory is 40-160%. 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 generally considered to be the single most important test for von Willebrand
Disease. However, complete evaluation of a patient requires determination
of other factors including von Willebrand Factor Antigen, von Willebrand Factor
Multimers, Factor VIII activity, Platelet Function Analysis (PFA-100), blood
group and family history.