Blood Test: Factor VIII Activity



Blood Test: Factor VIII Activity
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 12 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, 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
short-sleeve shirt to allow easier access for 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
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 action.

Risks



The factor VIII activity test is considered a safe procedure.
However, as with many medical tests, some problems can occur with having
blood drawn:

• 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 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 factor VIII activity test, speak with
your doctor. You can also talk to the technician before the procedure.