Factor II assay

Alternative names:

thrombin

What the risks are:

• excessive bleeding
• fainting or feeling lightheaded
• 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 slower 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.