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Fibrin-Stabilizing Factor (FSF) Blood Test

Last updated May 9, 2019

Approved by: Krish Tangella MD, MBA, FCAP

The Fibrin-Stabilizing Factor (FSF) Blood Test is performed to determine if an individual has deficient or decreased levels of Factor XIII.

What are the other Names for this Test? (Equivalent Terms)

  • FSF Blood Test
  • Factor XIII (Fibrin-Stabilizing Factor) Coagulation Blood Test

What is Fibrin-Stabilizing Factor (FSF) Blood Test? (Background Information)

  • The Fibrin-Stabilizing Factor (FSF) Blood Test is performed to determine if an individual has deficient or decreased levels of Factor XIII
  • The FSF Blood Test is a very uncommon laboratory test. Many laboratories do not have the capability to perform this test. Only highly-specialized labs that perform advanced blood clotting testing procedures perform this test
  • Clotting factors are proteins that help form blood clots at the site of blood vessel injury. Blood clots stop blood loss and allow the blood vessel to continue functioning
  • The clotting of blood may be affected due to the absence of clotting Factor XIII, decreased levels of Factor XIII, or abnormal function of Factor XIII. Typically, the bleeding is proportional to Factor XIII levels in the body; more Factor XIII implies less bleeding possibility, and less Factor XIII implies more bleeding possibility
  • Factor XIII deficiency is a very rare autosomal recessive disorder. This condition is very common in regions of the world where there is marriage among close blood relatives
  • Injury to a blood vessel causes the blood vessel to constrict. Called the vascular phase, this is the first reaction of a blood vessel to damage. It reduces the flow of blood to the site of injury, minimizing blood loss
  • Next, circulating platelets clump along the site of blood vessel injury. Platelets form a foundation for a blood clot and release chemicals that stimulate clotting
  • The coagulation phase then causes a blood clot to form. Clotting occurs when an enzyme called thrombin converts a soluble protein, fibrinogen, into its insoluble form, fibrin. Fibrin proteins make up the bulk of a blood clot
  • Thrombin is activated by the merging of two pathways, called the intrinsic and extrinsic pathways, into the common pathway. These are initiated by different parts of the body after blood vessel damage:
    • The intrinsic pathway begins in the blood with the activation of circulating proteins
    • The extrinsic pathway begins in the blood vessel with the release of protein factors by damaged cells lining the vessel
  • The extrinsic pathway is the first to activate. The intrinsic pathway then reinforces the extrinsic pathway and provides longer-lasting clotting effects
  • Coagulation factors are central to the action of these pathways. Each factor activates the next in a stepwise fashion
  • Once a coagulation factor is activated, it remains active. Thus, with each step in the pathway, more and more factors are activated. This results in a cascade of events similar to the snowball effect
  • A counter pathway ensures that the size of the growing blood clot stays in check. Problems with this regulatory pathway may lead to a dangerous condition where a blood clot forms within blood vessels (thrombosis)
  • The Coagulation Factors Blood Tests are tests to help assess the activity and levels of 9 coagulation factors (though there are 12 total). These tests are used to investigate bleeding disorders, especially those that are genetic disorders

What are the Clinical Indications for performing the Fibrin-Stabilizing Factor (FSF) Blood Test?

Following are the clinical indications for performing the Fibrin-Stabilizing Factor Blood Test:

  • Following up to a prothrombin time (PT) or partial thromboplastin time (PTT) blood test
  • Bleeding related signs and symptoms: Frequent bleeding from the nose, blood in stool and urine, easy bruising, frequent bleeding from the gums, and frequent miscarriages. However, the signs and symptoms are related to the levels of Factor XIII in the body
  • Before undergoing an elective surgery, if there is a history of bleeding tendency; or, if there is uncontrolled or excessive bleeding after a surgery
  • Family history of coagulation Factor XIII deficiency

How is the Specimen Collected for Fibrin-Stabilizing Factor (FSF) Blood Test?

Following is the specimen collection process for Fibrin-Stabilizing Factor Blood Test:

Sample required: Blood

Process of obtaining blood sample in adults:

  • A band is wrapped around the arm, 3-4 inches above the collection site (superficial vein that lies within the elbow pit)
  • The site is cleaned with 70% alcohol in an outward spiral, away from the zone of needle insertion
  • The needle cap is removed and is held in line with the vein, pulling the skin tight
  • With a small and quick thrust, the vein is penetrated using the needle
  • The required amount of blood sample is collected by pulling the plunger of the syringe out slowly
  • The wrap band is removed, gauze is placed on the collection site, and the needle is removed
  • The blood is immediately transferred into the blood container, which has the appropriate preservative/clot activator/anti-coagulant
  • The syringe and the needle are disposed into the appropriate “sharp container” for safe and hygienic disposal

Preparation required: No special preparation is needed prior to the test.

What is the Significance of the Fibrin-Stabilizing Factor (FSF) Blood Test Result?

Low levels of coagulation Factor XIII may indicate:

  • A congenital deficiency of Factor XIII
  • Decreased production in the body of Factor XIII, due to a variety of reasons

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

  • Coagulation factors were named in the order of their discovery. For example, Factor I was the first factor discovered. Their names have nothing to do with the order in which they act during the clotting process
  • Factor VI was later proven to be a modified version of Factor V. Thus, the name Factor VI is no longer used

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

Please visit our Laboratory Procedures Center for more physician-approved health information:


References and Information Sources used for the Article:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: May 31, 2015
Last updated: May 9, 2019