What It Is
A von Willebrand factor (vWF) activity – ristocetin cofactor test lets doctors
evaluate the functioning of the protein vWF, which helps blood to clot. A clot
is a lump of blood that the body produces to prevent excessive bleeding by sealing
leaks from blood vessels caused by wounds, cuts, scratches, or 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 vWF,
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 called 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 activity – ristocetin cofactor 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 may also 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)
- the vWF antigen test (which checks blood levels of vWF)
- 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.