Also Known As
Factor I Assay
Fibrinogen Activity (Functional)
Fibrinogen Antigen
Cardiac Fibrinogen
Formal Name
Fibrinogen Activity and Fibrinogen Antigen Assays
This article was last reviewed on
This article waslast modified on May 29, 2019.
At a Glance
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 (known as coagulation factor I); sometimes used to help evaluate your risk of developing cardiovascular disease

When To Get Tested?

When you have excessive bleeding (e.g., easy bruising, frequent nosebleeds, bleeding gums), a harmful blood clot, or both (e.g., DIC); when your prothrombin time (PT) and/or partial thromboplastin time (PTT) is prolonged; when you have a relative with a hereditary fibrinogen deficiency or abnormality; sometimes when your healthcare practitioner wants additional information to help evaluate your risk of heart disease

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

You may be able to find your test results on your laboratory's website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab's website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab's website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

What is being tested?

Fibrinogen is a protein, specifically a clotting factor (factor I), that is essential for proper 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. A fibrinogen antigen test measures the amount of fibrinogen in the blood.

Fibrinogen is produced by the liver and released into the blood along with several other clotting factors (also called coagulation factors). 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 cellular fragments called platelets clump and stick to the injury site and clotting factors are activated one after the other. This last part is called the coagulation cascade.

As the cascade nears completion, soluble fibrinogen is converted into insoluble fibrin threads. These threads then 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 is healed.

For a stable clot to form, there must be enough normally functioning platelets and coagulation factors. Too little, too much, or dysfunctional clotting factors or platelets can lead to bleeding episodes and/or to the formation of an inappropriate blood clot (thrombosis). Several laboratory tests, including fibrinogen tests, can be used to evaluate the clotting process. Fibrinogen testing includes:

  • A fibrinogen activity test evaluates that part of the clotting process in which soluble fibrinogen is converted into fibrin threads. It measures the time that it takes for a fibrin clot to form after a standard amount of thrombin is added to your blood sample (plasma). The time it takes 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 levels of normal fibrinogen or may be due to fibrinogen that does not function as it should (dysfunctional).
  • A fibrinogen antigen test measures the level of fibrinogen in a blood sample, including both functional and dysfunctional fibrinogen.

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.

Accordion Title
Common Questions
  • How is it used?

    This testing is used to evaluate fibrinogen (factor I), a protein that is essential for blood clot formation.

    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:
    • A fibrinogen antigen test is occasionally used as a follow-up test to determine whether decreased fibrinogen activity is due to insufficient fibrinogen or dysfunctional fibrinogen.
  • When is it ordered?

    A healthcare practitioner may order a fibrinogen activity test when you have:


    A fibrinogen antigen test may be performed when you have an abnormally low result on a fibrinogen activity test to help determine whether it is due to low fibrinogen or fibrinogen that does not function properly.

    In some cases, fibrinogen testing may be performed along with other cardiac risk markers when your healthcare practitioner wants to have additional information to help evaluate your risk of heart 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. Results are usually interpreted in the context of results from other tests.

    Normal fibrinogen activity results usually reflect normal contribution of fibrinogen to your blood clotting ability.

    Significantly decreased fibrinogen activity may be due to a decreased amount of fibrinogen or fibrinogen that does not function as it should. Reduced fibrinogen activity and antigen levels may affect your ability to form a stable blood clot.


    Sometimes a healthcare practitioner will use a ratio of the antigen test and the activity test. This is to help to distinguish dysfunctional fibrinogen (high ratio) from low fibrinogen (ratio close to 1).

    Fibrinogen is an acute phase reactant, meaning that fibrinogen levels may rise sharply in any condition that causes inflammation or tissue damage. High levels of fibrinogen are not specific. They do not tell the healthcare practitioner the cause or location of the inflammation or damage. Usually these increased levels are temporary, returning to normal after the underlying condition has resolved.

    Increased fibrinogen levels may be seen with:


    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 cardiovascular disease.

  • What are congenital (inherited) dysfibrinogenemia, hypofibrinogenemia and afibrinogenemia?

    These are a rare inherited coagulation disorders caused by changes (mutations) in the genes controlling the production of fibrinogen in the liver.

    • Congenital dysfibrinogenemia causes the liver to make an abnormal, dysfunctional fibrinogen, one that resists degradation when converted to fibrin or cannot function normally in the coagulation cascade. Dysfibrinogenemia may increase a person's risk of a blood clot or, rarely, cause a mild bleeding tendency. People with fibrinogen deficiency or dysfibrinogenemia may experience poor wound healing.
    • Congenital hypofibrinogenemia results in decreased fibrinogen. People with this condition can experience mild bleeding episodes, such as a bloody nose or bleeding gums.
    • Congenital afibrinogenemia is a severe lack of fibrinogen. People with this condition may be at risk of severe bleeding (hemorrhaging) episodes, especially as babies or as young children. They may experience excessive bleeding from the umbilical cord, frequent, easy bruising, nose bleeds that are difficult to stop, excessive bleeding after surgical procedures, and bleeding in the digestive tract.

    Genetic testing is occasionally performed for people with these inherited disorders to identify the genetic mutation responsible. Testing for this mutation may also be performed for other family members.

  • If my fibrinogen level is increased, what can I do to reduce it?

    If your fibrinogen concentration is elevated due to an acute inflammatory process, it will likely return to normal 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 healthcare 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 LDL cholesterol and raising your HDL cholesterol. There is also some evidence that diets rich in omega-3 and omega-6 fatty acids (fish oils) may help reduce fibrinogen levels.

  • 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 fragments of the dissolving clot, while D-dimer is a more specific measurement for one of the final crosslinked, break-down fragments.

  • 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.

  • 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 plasminogen activators (t-PA), and valproic acid. Moderate elevations in fibrinogen are sometimes seen with pregnancy, cigarette smoking, and with oral contraceptives or estrogen use.

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