The Test Sample
What is being tested?
Von Willebrand factor (vWF or VWF) is a protein that is one of several components of the coagulation system that work together, and in sequence, to stop bleeding within the body. VWF testing measures the amount of the protein present in blood and determines how well the protein functions.
Normally, when a blood vessel is damaged and bleeding begins, VWF forms an adhesive bridge between activated cell fragments called platelets and the injury site. This is followed by the clumping (aggregation) of platelets at the site and a series of actions referred to as activation of the coagulation cascade, resulting in the formation of a stable blood clot.
VWF further affects clotting by influencing the availability of coagulation factor VIII. VWF carries factor VIII in the blood, increases its half-life, and releases it as necessary. If the amount of functional VWF is insufficient, then platelet adherence and aggregation are affected, levels of factor VIII could be decreased, blood clot formation takes longer, and therefore bleeding is prolonged. This deficiency causes a condition referred to as von Willebrand disease (VWD).
VWD is the most common inherited bleeding disorder. It is a group of conditions associated with prolonged bleeding due to deficient and/or defective VWF. VWD is separated into different types and sub-types, including:
- Type 1 – with this type of VWD, there is a decrease in the amount of VWF produced, but the VWF functions normally. Levels of factor VIII are also typically low but may be normal. This is the most common type of VWD, accounting for about 75% of cases. It tends to cause bruising and mild to moderate bleeding, such as persistent nosebleeds, heavy menstrual periods, and prolonged bleeding following childbirth, trauma, dental procedures, and surgeries. Symptoms and the severity of bleeding will vary from person to person and from episode to episode.
- Type 2 – this type is associated with a normal amount of VWF, but the VWF does not function normally. Bleeding may be more severe with this with this type. Type 2 is further divided into Types 2A, 2B, 2M, and 2N.
- Type 3 – this rare type is associated with very little VWF production, very low factor VIII levels, and moderate to severe symptoms. It is often detected in infancy because of early bleeding episodes.
Rarely, VWD may be due to an acquired VWF deficiency, where there is no family or personal bleeding history up to the point when signs and symptoms develop. (For more on this, see Common Questions #7.)
Von Willebrand factor testing includes VWF antigen, which measures the amount of VWF, and VWF activity (also known as Ristocetin Cofactor), which evaluates the function of VWF. Some laboratories may offer a panel that includes both of these tests along with a factor VIII activity test.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.