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Factor II assay


The factor II assay is a blood test to measure the activity of factor II. Factor II is also known as prothrombin. This is one of the proteins in the body that helps the blood clot.

Alternative Names

Prothrombin assay

How the Test is Performed

A blood sample is needed.

How to Prepare for the Test

No special preparation is needed.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

Why the Test is Performed

This test is used to find the cause of too much bleeding (decreased blood clotting). This decreased clotting may be caused by an abnormally low level of factor II.

Normal Results

The value should be 50 to 200% of the laboratory control or reference value.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different samples. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

Decreased factor II activity may be the result of:


Veins and arteries vary in size so it may be harder to take a blood sample from one person than another.

Other slight risks associated with having blood drawn may include:

  • Excessive bleeding
  • Fainting or feeling light headed
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)

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.


Schmaier AH. Laboratory evaluation of hemostatic and thrombotic disorders. In: Hoffman R, Benz EJ Jr, Silberstein LE, Heslop HE, Weitz JI, Anastasi J, eds. Hematology: Basic Principles and Practice. 6th ed. Philadelphia, PA: Elsevier Saunders; 2013:chap 131.

Review Date: 1/27/2015
Reviewed By: Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital, Boston, MA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.
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