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.
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
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 compound in the vial keeps the blood from clotting before the sample is analyzed.
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 few days.
Getting the Results
At the lab, factor VIII activity is determined through a clotting-time test. First
the blood cells are separated from the plasma (the liquid part of the blood). Then
the technician adds to the sample plasma some additional plasma that has been depleted
of factor VIII. The clotting time for this mixture is then compared with the clotting
time of normal plasma.
Tests results, which are usually available after a few days, are reported as the
patient's percentage of the factor VIII activity in normal plasma. A low percentage
is seen with hemophilia A, though the condition may be mild or severe.
Low levels also may indicate the presence of factor VIII inhibitors, which are
antibodies that some kids with severe hemophilia develop when their bodies react to
the clotting factor as a foreign substance and create antibodies to block its clotting
The factor VIII activity test is considered a safe procedure. However, as with
many medical tests, some problems can occur with having blood drawn. These include:
fainting or feeling lightheaded
hematoma (blood accumulating under the skin causing a lump or a bruise)
pain associated with multiple punctures to locate a vein
Helping Your Child
Having a blood test is relatively painless. Still, many kids are afraid of needles.
Explaining the test in terms your child can understand might help ease some of the
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 factor VIII activity test, speak with your doctor.
You also can talk to the technician before the procedure.