What It Is
A von Willebrand factor (vWF) antigen test measures the quantity of a protein called von Willebrand factor that helps blood to clot. A clot is a lump of blood that the body produces to prevent excessive bleeding by sealing leaks in blood vessels caused by wounds, cuts, scratches, and other conditions.
The blood's ability to clot is a complex process involving platelets (also called thrombocytes) and proteins called clotting factors. Platelets are oval-shaped cells made in the bone marrow. Most clotting factors are made in the liver. Some, like factor VIII and vWF, which circulate in the body bound to one another, are made in blood vessel walls.
When a blood vessel breaks, platelets are first to the area to help seal the leak and temporarily stop or slow bleeding. But for the clot to become strong and stable, the action of clotting factors is required.
The body's clotting factors are numbered using the Roman numerals I through XII. They work together in a specialized sequence, almost like pieces of a puzzle. When the last piece is in place, the clot develops — but if even one piece is missing or defective, the puzzle can't come together.
Von Willebrand factor is involved in the early stages of blood clotting. As the platelets gather at the injury site, vWF acts like glue, helping them stick together to stop the bleeding.
Sometimes, though, children are born with an abnormal gene that causes them to produce too little vWF, or a defective version of it. The result is a bleeding disorder known as von Willebrand disease.
Variations of von Willebrand disease range from mild to severe. In most cases, the genetic mutation that causes the disease is hereditary, but it may occur spontaneously as well.
Why It's Done
Doctors order the vWF antigen test to help diagnose or monitor the treatment of von Willebrand disease. Symptoms of von Willebrand disease can include easy bruising, frequent nosebleeds, excessive bleeding after a mouth injury or dental work, abnormal menstrual bleeding, or, in infant boys, prolonged bleeding after circumcision.
Von Willebrand disease also may be suspected when a child has a family member with a bleeding disorder.
This test is frequently performed along with other tests that give doctors a fuller picture of clotting ability. These may include:
- clotting time tests (such as prothrombin time [PT] and partial thromboplastin time [PTT])
- vWF activity – ristocetin cofactor (which measures how well the von Willebrand factor is working)
- the factor VIII activity test (which checks the amount and functioning of factor VIII, the factor that's missing or defective in the bleeding disorder hemophilia A)
Because factor VIII circulates in the body attached to vWF, a decreased amount of factor VIII can also mean a decreased amount of vWF.
Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor.
