Factor X assay

Definition:

A test to measure the activity of factor X (one of the substances used for coagulation) in the blood.

How the test is performed:

Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test:

Adults:
Your health care provider may ask you to withhold certain medications. (See "Special considerations".)

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

infant test or procedure preparation (birth to 1 year)
toddler test or procedure preparation (1 to 3 years)
preschooler test or procedure preparation (3 to 6 years)
schoolage test or procedure preparation (6 to 12 years)
adolescent test or procedure preparation (12 to 18 years)

How the test will feel:

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed:

This test may be used to detect the specific cause of excessive bleeding (decreased blood clotting).

Normal Values:

A normal value is 50 to 200% of the laboratory "control" or reference value.

What abnormal results mean:

Decreased factor X activity may indicate:

congenital deficiency of factor X
fat malabsorption (steatorrhea)
heparin administration
liver disease
Vitamin K deficiency
warfarin administration

What the risks are:

excessive bleeding
fainting or feeling light-headed
hematoma (blood accumulating under the skin)
infection (a slight risk any time the skin is broken)
multiple punctures to locate veins

Note: This test is most often performed on people who have bleeding problems. The risk of excessive bleeding is slightly greater than for people without bleeding problems.

Special considerations:

Coagulation (blood clotting) results from a sequence (cascade) of reactions involving the coagulation factors. Some of these factors have other names, for example, Factor I (fibrinogen), Factor II (prothrombin), and Factor XII (Hageman factor). These proteins are produced in the liver and secreted into the blood. Some of the Factors (that is, II, VII, IX, and X) require Vitamin K for their synthesis. Warfarin (Coumadin) is a commonly-used "anticoagulant" drug. It acts in the liver by inhibiting the enzyme that requires Vitamin K.

The coagulation sequence is initiated when some of the coagulation factors contact damaged tissue. Each coagulation factor reaction triggers the next reaction in the cascade. The final product of the coagulation cascade is the fibrin clot (blood clot).

Substances that inhibit the action of the coagulation factors, and plasmin, which eventually lyses (breaks down) the fibrin clot, are activated by the damaged tissue at the same time as the coagulation factors. However, they function more slowly and over a longer period of time than the coagulation factors. This allows a clot to form to stop bleeding, then the clot is dissolved (after enough time for the tissue to heal) to restore blood flow. The most important of the coagulation inhibitors is antithrombin III, a protein that requires endogenous (made in the body) heparin for its activity.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Review Date: 11/10/2001
Reviewed By: Rebecca Elstrom, M.D., Division of Hematology-Oncology, University of Pennsylvania Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network.

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