Fibrin Split Products (FSPs), also known as Fibrin Degradation Products (FDPs), are protein fragments that circulate in the bloodstream after a blood clot has been broken down. When the body forms a clot, it uses the protein fibrin to create a stabilizing mesh. FSPs are the leftover fragments after this mesh is dissolved. Measuring the concentration of these fragments provides medical professionals with an indirect marker of biological activity, suggesting the body is attempting to clear significant or widespread clot formation.
The Biology of Fibrin and Clot Dissolution
The process of forming a stable blood clot begins when an injury triggers the conversion of the soluble protein fibrinogen into insoluble fibrin. The enzyme thrombin initiates this reaction, causing fibrin molecules to link together and form a dense, three-dimensional meshwork that traps blood cells and platelets. This fibrin mesh provides the structural integrity necessary to stop bleeding, a process called coagulation.
Once the injury has healed and the clot is no longer needed, the body initiates a controlled breakdown process known as fibrinolysis. The central player in this process is the enzyme plasmin, which is activated from its inactive form, plasminogen, and acts as a molecular scissor. Plasmin systematically cuts the fibrin mesh into smaller, soluble fragments.
These resulting protein fragments, which include various components labeled as X, Y, D, and E, are collectively known as FSPs. These products are normally cleared from the circulation by the liver and kidneys. However, their concentration rises significantly when the rate of clot breakdown increases, indicating activation of the fibrinolytic system.
Measuring Fibrin Split Products
FSPs are measured using a blood test, often called the FDP assay. This test is ordered when a healthcare provider suspects a patient may be experiencing a condition involving excessive or abnormal blood clotting and subsequent clot breakdown. The FDP assay typically uses a sample of the patient’s blood serum or plasma to detect and quantify these protein fragments.
In a healthy individual, the concentration of FSPs in the blood is expected to be very low because the body’s clotting and dissolving processes are balanced. Most laboratories consider a normal reference range for FSPs to be less than 10 micrograms per milliliter (mcg/mL), though some may report less than 5 mcg/mL. Levels above this range suggest an overactive fibrinolytic system.
The FDP assay is a useful tool for screening, but its results must be interpreted alongside other laboratory measurements and the patient’s overall condition. Because FSPs can be elevated due to various non-clotting conditions, such as liver disease or massive trauma, the test provides evidence of fibrinolytic activity rather than confirming a specific diagnosis on its own.
Clinical Significance of Elevated FSPs
An elevated FSP level indicates that the body is simultaneously clotting blood and actively dissolving those clots. This dual activity points toward a severe underlying condition that is causing widespread thrombus formation. This interpretation suggests a disruption to the normal balance of the coagulation system.
The most recognized and serious condition strongly associated with extremely high FSPs is Disseminated Intravascular Coagulation (DIC). DIC is a life-threatening disorder where the coagulation system becomes systemically overactive, leading to the formation of small clots throughout the body’s blood vessels, which then rapidly activate the fibrinolytic system. FSP levels above 40 µg/mL are highly indicative of overt DIC, and monitoring these levels can help track the disorder’s severity and the patient’s response to treatment.
Elevated FSPs also serve as markers for other thromboembolic disorders and conditions causing acute tissue damage or inflammation. These conditions include:
- Deep Vein Thrombosis (DVT), a clot in a deep vein.
- Pulmonary Embolism (PE), where a clot lodges in the lung’s arteries.
- Severe liver disease.
- Acute myocardial infarction (heart attack).
- Preeclampsia.
D-dimer is a more specific type of FSP that is often measured instead. FSPs result from the breakdown of both fibrinogen and non-cross-linked fibrin. D-dimer specifically results from the breakdown of cross-linked fibrin, which is the stable form found in an established blood clot. Consequently, D-dimer is considered more specific for confirming the presence of a clot that has formed and stabilized in the body, whereas FSPs are a broader indicator of general fibrinolytic activity.