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.