Factor X Assay

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

What Is a Factor X Assay Test?

When you bleed, a cascade of reactions takes place in your body to stop or prevent blood loss so that the damaged tissue can be repaired. This process is called coagulation.

Coagulation factors, or clotting factors, are the substances in your blood that help a clot form. There are 13 factors that control clotting. A deficiency in any one of them will compromise the clotting process and can lead to excessive bleeding. Factor X is one of these coagulation factors.

Your doctor may recommend a factor X assay test to determine if your body makes enough of this particular coagulation factor. Without it, your body will be unable to form blood clots normally.

Women with factor X deficiency may suffer from heavy menstrual periods and bleeding after childbirth. Newborn boys with inherited factor X deficiency may suffer from prolonged bleeding after circumcision.

Emergency dental work or surgery may cause severe bleeding and alert you to a potential problem. It is important to see your doctor for a diagnosis if you have any blood clotting problems.

What Causes a Factor X Deficiency?

An inherited genetic defect sometimes causes a factor X deficiency. This is known as inherited factor X deficiency. According to the NIH, it affects only one out of every 500,000 people (NIH, 2011). Episodes of bleeding may range from mild to severe.

Acquired factor X deficiency is more common. The condition is not genetic and is not present at birth. Underlying conditions such as a vitamin K deficiency, or the use of medications that prevents blood clotting such as warfarin (Coumadin) may cause it.

How Is the Test Administered?

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

Your doctor will need to take a sample of blood from your arm to perform the test. 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 X assay should be between 60 and 140 percent of the laboratory reference value. Your doctor will explain the specifics of your results.

Abnormal Results

If your results are abnormal, you have a low level of factor X. This could be caused by:

  • an inherited factor X deficiency (also known as Stuart-Prower factor deficiency)
  • amyloidosis (an abnormal deposit of protein in your tissues and organs)
  • disseminated intravascular coagulation (a disease in which the proteins responsible for blood clotting are abnormally active)
  • a vitamin K deficiency
  • fat malabsorption
  • the use of heparin or warfarin blood thinners

What Are the Risks of the Test?

As with any blood test, there is a slight risk of bruising, bleeding, or infection 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 to the site 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

Bleeding caused by factor X deficiency can be controlled with regular infusions of fresh frozen blood plasma or the injection of concentrated clotting factors into your blood. If the disorder is caused by a lack of vitamin K, you can take vitamin K supplements orally or as an injection.

The long-term outlook for factor X deficiency will vary depending on the severity of your condition. If you have a mild case, you may experience few symptoms; however, more severe cases can be fatal.


Rare complications from factor X deficiency may include:

  • death from severe blood loss
  • joint deformity due to frequent bleeding into the joint cavities
  • miscarriage during the first trimester of pregnancy
  • spontaneous bleeding inside the head, skull, or digestive tract
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