Factor VII assay

Definition

Factor VII assay is a blood test to measure the activity of factor VII -- one of the substances used for coagulation.

How the test is performed

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell 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 band 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.

In infants or young children:

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. A bandage may be applied to the puncture site if there is any bleeding.

How to prepare for the test

Your health care provider may ask you to stop taking certain medications.

How the test will feel

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

Why the test is performed

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

Normal Values

The normal value is 50-200% of the laboratory control or reference value.

What abnormal results mean

Decreased factor VII activity may be associated with:

• Congenital deficiency of factor VII
• Fat malabsorption
• Liver disease (such as cirrhosis)
• Vitamin K deficiency
• Warfarin administration
• Disseminated intravascular coagulation (DIC)

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

Special considerations

Coagulation (blood clotting) results from a sequence of reactions involving several proteins known as coagulation factors. Some of these factors have other names. For example, factor I is also called fibrinogen, factor II is prothrombin, and factor XII is Hageman factor. The liver produces these proteins and secretes them into the blood. In addition, vitamin K is important to blood clotting because your body converts it into prothrombin.

Some people take warfarin to keep their blood from clotting. Warfarin inhibits prothrombin, thus interrupting the clotting cascade. Because of the link between vitamin K and prothrombin, people who take warfarin need to have consistent levels of vitamin K in their diet, as instructed by their doctor.

Coagulation begins when some of the coagulation factors contact damaged tissue. Each factor reaction triggers the next reaction, in a cascade. The final product of the coagulation cascade is the blood clot.

Substances that inhibit or turn off the action of the coagulation factors are activated along with the coagulation factors. These substances function more slowly, over a longer period of time, than the coagulation factors. This allows a clot to form, stopping the bleeding, then, after enough time has passed for the tissue to heal, the clot is dissolved and blood flow restored. The most important coagulation inhibitor is antithrombin III.

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.