en españolAnálisis de sangre: antígeno del factor von Willebrand (vWF)

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.

Preparation

No special preparations are needed for this test. Tell the doctor if your child takes any blood-thinning medications, because these may affect the results. On the day of the test, having your child wear a T-shirt or short-sleeved shirt can make things easier for your child and the technician who will be drawing the blood.

The Procedure

A health professional will usually draw the blood from a vein. If the blood is being drawn from a vein, the skin surface is cleaned with antiseptic, and an elastic band (tourniquet) is placed around the upper arm to apply pressure and cause the veins to swell with blood. A needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial. A chemical in the vial keeps the blood from clotting before the test begins.

After the procedure, the elastic band is removed. Once the blood has been collected, the needle is removed and the area is covered with cotton or a bandage to stop the bleeding. Collecting blood for this test will only take a few minutes.

What to Expect

Collecting a sample of blood is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a day or so.

Getting the Results

At the lab, the blood cells are separated from the plasma (the liquid part of the blood). The test measures the total amount of vWF protein in the plasma.

Tests results, which are usually available after several days, are reported as a percentage of what would be expected in normal plasma. 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.

Risks

The vWF antigen test is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn, such as:

• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin causing a lump or bruise)
• pain associated with multiple punctures to locate a vein

Helping Your Child

Having a blood test is relatively painless. Still, many children are afraid of needles. Explaining the test in terms your child can understand might help ease some of the fear.

Allow your child to ask the technician any questions he or she might have. Tell your child to try to relax and stay still during the procedure, as tensing muscles and moving can make it harder and more painful to draw blood. It also may help for your child to look away when the needle is being inserted into the skin.

If You Have Questions

If you have questions about the vWF antigen test, speak with your doctor. You can also talk to the technician before the procedure.