What It Is
A factor VIII activity blood test lets doctors evaluate the functioning of a protein that 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.
Blood clotting is a 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. 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.
Factor VIII, with factor IX, is involved in the last step of the clotting process — the creation of a "net" that closes a torn blood vessel. When an abnormal gene causes a child to be deficient in factor VIII, the result is a bleeding disorder known as hemophilia A. A factor IX deficiency is known as hemophilia B. Both conditions are usually hereditary, but also can occur spontaneously.
Why It's Done
Doctors order the factor VIII activity test to help diagnose or monitor the treatment of hemophilia A. Signs or symptoms of hemophilia can include easy bruising, nosebleeds that won't stop, excessive bleeding after a mouth injury or dental procedure, bleeding gums, blood in the urine, or swollen or painful joints.
The factor VIII activity test also may be done to help identify the reason for an abnormal result on other clotting tests (such as prothrombin time [PT] or partial thromboplastin time [PTT]), or when a child has a family member with a bleeding disorder.
It also may be done as part of an evaluation for a bleeding disorder called von Willebrand disease. Because factor VIII circulates in the body attached to another clotting factor called von Willebrand factor (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, as these may affect the results.
On the day of the test, it may help to have your child wear a T-shirt or short-sleeved shirt to allow easier access for the technician who will be drawing the blood.