Factor VIII Assay

Written by Corinna Underwood | Published on June 14, 2012
Medically Reviewed by George Krucik, MD

What is a Factor VIII Assay Test?

Your doctor may recommend a factor VIII assay test to determine whether your body produces a healthy level of this particular coagulation factor. Your body needs factor VIII in order to form blood clots.

Each time you bleed, it triggers a series of reactions known as the “coagulation cascade.” Coagulation is the process your body uses to stop blood loss. Cells called platelets create a plug to cover the damaged tissue, and then your body’s 13 clotting factors interact to produce a blood clot. Low levels of any of these clotting factors can prevent a clot from forming.

What the Test Addresses

This test is used to determine the cause of prolonged or excessive bleeding. Your doctor may recommend the test if you have a family history of bleeding disorders or if you have experienced any of the following symptoms:

  • abnormal or excessive bleeding
  • easy bruising
  • heavy or prolonged menstrual periods
  • frequent gum bleeding
  • frequent nosebleeds

This test may also be ordered if you doctor believes you have an acquired condition that is causing your bleeding disorder, such as:

  • vitamin K deficiency
  • liver disease

This assay can also determine whether you are suffering from an inherited factor VIII deficiency, particularly if you have been experiencing bleeding episodes since you were a child or if a member of your family has an inherited factor deficiency. In the latter case, your close relatives may also be tested to help confirm your diagnosis.

An inherited factor VIII deficiency is called hemophilia A. This hereditary condition only affects males because it is linked to a defective gene on the X chromosome. Females have two X chromosomes, so if one has a defective gene, the other can still create enough factor VIII.

If you have already been diagnosed with a factor VIII deficiency and are undergoing treatment, your doctor may order this test to determine the effectiveness of your treatment.

Preparing for the Test

No special preparation is necessary for this test. You should tell your doctor if you are taking any blood-thinning medications, such as warfarin (Coumadin). Your doctor may advise you to stop taking blood thinners before the test.

How the Test Is Administered

To perform the test, your doctor will need to take a sample of blood from your arm. First, the site will be cleaned with an alcohol swab. Then, the doctor will insert a needle into your vein and attach a tube to the needle to collect blood. When enough blood has been collected, the needle will be removed and the site covered with a gauze pad. The blood sample will be sent to a laboratory for analysis.

Understanding the Results

Normal Results

A normal result for a factor VIII assay should be around 100 percent of the laboratory reference value. Your doctor will explain the specifics of your results.

Abnormal Results

If you have an abnormally low level of factor VIII, it could be caused by:

  • an inherited factor VIII deficiency (hemophilia A)
  • disseminated intravascular coagulation (a disease in which the proteins responsible for blood clotting are abnormally active)
  • the presence of a factor VIII inhibitor
  • Von Willebrand’s disease (a disorder that causes abnormal blood clotting)

If you have an abnormally high level of factor VIII, it may be caused by:

  • diabetes
  • pregnancy
  • advanced age
  • inflammation
  • obesity
  • liver disease

What Are the Risks of the Test?

As with any blood test, there is a slight risk of bruising or bleeding at the puncture site. In rare cases, the vein may become swollen after blood is drawn. This condition, known as phlebitis, can be treated by applying a warm compress several times a day.

Ongoing bleeding could be a problem if you suffer from a bleeding disorder or are taking blood-thinning medication, such as warfarin or aspirin.

What to Expect After the Test

If you have been diagnosed with a factor VIII deficiency, your doctor will prescribe replacement concentrates of factor VIII. The amount you require will depend on:

  • your age
  • your height and weight
  • the severity of your bleeding
  • the site of your bleeding

To help prevent a bleeding emergency, your doctor will teach you and your family how to administer factor VIII concentrates at home at the first sign of bleeding.

If your levels of factor VIII are too high, you are at a higher risk for thrombosis (an obstructive blood clot in your vein). In this case, your doctor may perform additional tests or prescribe anticoagulants.

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