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.


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.

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.


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 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 also can talk to the technician before the procedure.

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Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.