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Also known as: Factor I; Fibrinogen Activity; Functional Fibrinogen; Fibrinogen Antigen; Plasma Fibrinogen; Hypofibrinogenemia Test
Formal name: Fibrinogen Activity and Fibrinogen Antigen Assays

The Test Sample

What is being tested?

Fibrinogen is a protein, a coagulation factor (factor I) that is essential for blood clot formation. Two types of tests are available to evaluate fibrinogen: a fibrinogen activity test evaluates how well fibrinogen functions in helping to form a blood clot while a fibrinogen antigen test measures the amount of fibrinogen in the blood.

Fibrinogen is produced by the liver and released into circulation along with several other coagulation factor proteins. Normally, when a body tissue or blood vessel wall is injured, a process called hemostasis begins to help stop the bleeding by forming a plug at the injury site. Small cell fragments called platelets adhere to and aggregate at the site, a coagulation cascade begins, and clotting factors are activated one after the other.

As the cascade nears completion, soluble fibrinogen is converted into insoluble fibrin threads. These threads crosslink together to form a fibrin net that stabilizes at the injury site. The fibrin net adheres to the site of injury along with the platelets to form a stable blood clot. This barrier prevents additional blood loss and remains in place until the injured area has healed.

For a stable clot to form there must be enough normally functioning platelets and coagulation factors. If there are dysfunctional factors or platelets, or too little or too much of them, it can lead to bleeding episodes and/or to formation of an in appropriate blood clot (thrombosis). Several laboratory tests, including fibrinogen tests, can be used to evaluate hemostasis.

It is now understood that coagulation tests are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, the tests can be used to evaluate specific components of the hemostasis system. The fibrinogen activity test evaluates that part of the hemostatic process in which soluble fibrinogen is converted into fibrin threads. With the addition of thrombin to the test sample, the fibrinogen test bypasses the rest of the coagulation factors and focuses on the function of fibrinogen.

  • A fibrinogen activity test measures the time that it takes for a fibrin clot to form following the addition of a standard amount of thrombin to plasma. This test evaluates the function of fibrinogen, its ability to be converted into fibrin. The time that is required for a clot to form directly correlates with the amount of active fibrinogen that is present. Prolonged clot-formation times may be due to decreased concentrations of normal fibrinogen or due to dysfunctional fibrinogen.
  • A fibrinogen antigen test uses a fibrinogen antibody to bind to fibrinogen in a blood sample. This test allows the quantity, but not activity, of fibrinogen to be measured.

Fibrinogen is also one of several blood factors that are called acute phase reactants. Blood levels of fibrinogen along with other acute phase reactants rise sharply with conditions causing acute tissue inflammation or damage. Tests for these acute phase reactants, including fibrinogen, may be performed to determine the extent of inflammation in the body.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.