What Is a Von Willebrand Factor Antigen Test?
Von Willebrand factor is involved in a few stages of blood clotting.
Proteins called clotting factors are needed for blood to clot properly and help prevent too much bleeding. A von Willebrand factor (vWF) antigen test measures the amount of a clotting factor called von Willebrand factor.
The body's clotting factors work together in a special order, almost like pieces of a puzzle. When the last piece is in place, the clot develops. But if there aren't enough of them, or any of them don't work as they should, it can take longer than normal for blood to clot.
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.
A person with 20% to 40% of the normal amount of vWF would likely have a mild form of von Willebrand disease.
A person with less than 10% would have a more severe form.
Increased concentrations of VWF antigen and VWF activity are not considered diagnostic. VWF is one of many acute phase reactants. This means that levels will be temporarily increased with infections, inflammation, trauma, and with physical and emotional stressors. They are also increased with pregnancy and with the use of estrogen medications such as oral contraceptives.
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