Patient Test Information


Also known as:

Factor I; Fibrinogen Activity; Functional Fibrinogen; Fibrinogen Antigen; Plasma Fibrinogen; Hypofibrinogenemia Test

Formal name:

Fibrinogen Activity and Fibrinogen Antigen Assays

Related tests:

PT and INR, PTT, D-dimer, Coagulation Factors, Thrombin Time, hs-CRP

Why Get Tested?

As part of an investigation of a possible bleeding disorder or blood clot (thrombotic episode), particularly to evaluate the level and function of fibrinogen; sometimes used to help evaluate your risk of developing cardiovascular disease

When to Get Tested?

When you have bleeding or thrombotic episodes; when a PT and/or PTT test is prolonged; when you have a relative with a hereditary fibrinogen deficiency or abnormality; when your health care provider wants additional information to help evaluate your risk of developing heart disease

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


How is it used?

This testing is used to evaluate fibrinogen, a protein that is essential for blood clot formation. When there is an injury and bleeding occurs, the body forms a blood clot through a series of steps. In one of the last steps, soluble fibrinogen is converted into insoluble fibrin threads that crosslink together to form a net that stabilizes and adheres at the injury site until the area has healed.

Two types of tests are available:

  • A fibrinogen activity test measures the function of fibrinogen and its ability to be converted into fibrin. It is used:
    • As part of an investigation of a possible bleeding disorder or inappropriate blood clot formation (thrombotic episode)
    • As a follow-up to an abnormal bleeding disorder test (prothrombin time, PT or partial thromboplastin time, PTT) and/or an episode of prolonged or unexplained bleeding
    • Along with tests such as PT, PTT, platelet function tests, fibrin degradation products (FDP), and D-dimer to help diagnose disseminated intravascular coagulation (DIC) or abnormal fibrinolysis
    • Occasionally to help monitor the status of a progressive disease (such as liver disease) over time or, rarely, to monitor treatment of an acquired condition (such as DIC)
    • Sometimes along with other cardiac risk markers such as C-reactive protein (CRP) to help determine a person's overall risk of developing cardiovascular disease. This use of the test has not gained widespread acceptance though because there are no direct treatments for elevated levels. However, many health practitioners feel that fibrinogen activity measurements give them additional information that may lead them to be more aggressive in treating those risk factors that they can treat (such as unhealthy levels of cholesterol).
  • A fibrinogen antigen test is occasionally ordered as a follow-up test to determine whether decreased fibrinogen activity is due to insufficient fibrinogen or dysfunctional fibrinogen (caused by inherited or acquired dysfibrinogenemia).

When is it ordered?

A health practitioner may order a fibrinogen activity test when someone:

  • Has unexplained or prolonged bleeding
  • Has a thrombosis 
  • Has an abnormal PT and PTT test result
  • Has symptoms of or is undergoing treatment for DIC or abnormal fibrinolysis
  • May have an inherited or acquired coagulation factor (clotting protein) deficiency or dysfunction
  • Has an acquired bleeding disorder and the person's health practitioner wants to evaluate and monitor their clotting ability (over time)

A fibrinogen antigen test may be performed when someone has a low result on a fibrinogen activity test to help determine whether it is due to insufficient or dysfunctional fibrinogen.

High fibrinogen levels have also been associated with coronary heart disease, myocardial infarction, and peripheral arterial disease. In some cases, fibrinogen activity testing is performed along with other tests when a health practitioner wants to evaluate an individual's risk of developing cardiovascular disease.

What does the test result mean?

Fibrinogen test results are reported as the concentration of the protein in the blood. Fibrinogen activity tests are converted into concentrations for comparison with fibrinogen antigen results.

Normal fibrinogen activity results usually reflect normal blood clotting ability.

Significantly decreased fibrinogen activity may be due to decreased or dysfunctional fibrinogen. Reduced fibrinogen activity and antigen levels may impair the body's ability to form a stable blood clot.

Chronically low levels may be related to decreased production due to an inherited condition such as afibrinogenemia or hypofibrinogenemia or to an acquired condition such as end-stage liver disease or severe malnutrition.

Acutely low levels are often related to consumption of fibrinogen such as may be seen with disseminated intravascular coagulation (DIC) and abnormal fibrinolysis, which occurs when the body is overactive in clearing blood clots. Reduced fibrinogen levels may also occur following rapid, large-volume blood transfusions and in people who are malnourished.

Sometimes a health practitioner will use a ratio of the antigen test and the activity test. This is to help to distinguish dysfibrinogenemia (high ratio) from hypofibrinogenemia (ratio close to 1).

Fibrinogen is an acute phase reactant, meaning that fibrinogen concentrations may rise sharply in any condition that causes inflammation or tissue damage. Elevated concentrations of fibrinogen are not specific; that is, they do not tell the health practitioner the cause or location of the disturbance. Usually these elevations in the fibrinogen level are temporary, returning to normal after the underlying condition has been resolved. Elevated levels may be seen with:

  • Acute infections
  • Cancer
  • Coronary heart disease, myocardial infarction
  • Stroke
  • Inflammatory disorders (like rheumatoid arthritis and glomerulonephritis, a form of kidney disease)
  • Trauma
  • Cigarette smoking
  • Pregnancy
  • Peripheral artery disease

While fibrinogen levels are elevated, a person's risk of developing a blood clot may be increased and, over time, they could contribute to an increased risk for developing cardiovascular disease.

Is there anything else I should know?

Blood transfusions within the past month may affect fibrinogen test results.

Certain drugs may cause decreased levels, including anabolic steroids, phenobarbital, streptokinase, urokinase, L-asparaginase, tissue plasmogen activators, and valproic acid. Moderate elevations in fibrinogen are sometimes seen with pregnancy, cigarette smoking, and with oral contraceptives or estrogen use.

Dysfibrinogenemia is a rare coagulation disorder caused by mutations in the gene controlling the production of fibrinogen in the liver. It causes the liver to make an abnormal, dysfunctional fibrinogen, one that resists degradation when converted to fibrin or can not function normally in the coagulation cascade. Dysfibrinogenemia may increase a person's risk of venous thrombosis or, rarely, cause a mild bleeding tendency. People with fibrinogen deficiency or dysfibrinogenemia may experience poor wound healing.

Genetic molecular testing is occasionally performed for those with inherited dysfibrinogenemia, hypofibrinogenemia, or afibrinogenemia to identify the genetic mutation responsible. Testing for this mutation may also be performed for other family members.

People with liver disease may develop acquired dysfibrinogenemia that could contribute to bleeding or thrombosis.

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.

  1. What can I do to reduce my fibrinogen level?

    If your fibrinogen concentration is elevated due to pregnancy or to an acute inflammatory process, it will likely return to normal by itself once the underlying condition has resolved. If it is due to an acquired condition such as rheumatoid arthritis, there may be very little you can do to affect the level. If your health care provider has told you that elevated fibrinogen levels are increasing your risk of cardiovascular disease, you can make lifestyle changes that will affect other cardiac risk factors, such as reducing your cholesterol and raising your HDL. There is also some evidence that diets rich in omega-3 and omega-6 fatty acids (fish oils) may help reduce fibrinogen levels.

  2. What is the difference between tests for fibrinogen, d-dimer, and fibrin degradation products (FDP)?

    Fibrinogen activity testing evaluates the conversion of fibrinogen into fibrin; fibrinogen antigen testing measures the amount of soluble Factor I (dissolved in the blood) before it has been turned into insoluble fibrin and been crosslinked into a fibrin net. D-dimer and FDP testing both help evaluate the status of the fibrinolytic system, the body's ability to break blood clots apart when they are no longer needed so that they can be removed. FDP is a measurement of all of the fragments of the dissolving clot, while D-dimer is a more specific measurement for one of the crosslinked, break-down fragments.

  3. Can I have decreased or abnormal fibrinogen and not know it?

    Yes. Many people have relatively normal clotting even when fibrinogen concentrations and/or activity are low. Your condition may not be identified unless you bleed longer than expected after a surgical procedure or trauma or have coagulation-related testing performed for another reason, such as part of a pre-surgical screen.