What Are FDPs and How Do They Work?
Fibrin and fibrinogen degradation products (FDPs) are part of the secondary anticoagulant system. They are generated during fibrinolysis, specifically during the breakdown of microthrombi.
FDPs function as potent anticoagulants that:
- Inhibit thrombin;
- Suppress fibrin formation: fibrin monomers lose their ability to polymerize and become resistant to thrombin;
- Inhibit platelet aggregation.
Regulation of FDP Levels in Blood
Under normal conditions, the reticuloendothelial system (RES) clears intermediate coagulation products from circulation.
- When the RES is blocked, such as in amniotic fluid embolism, circulating FDP levels increase significantly.
- Newborns, particularly premature infants, have a reduced capacity to clear activated clotting factors and FDPs. This is due to immature phagocytosis and fibrinolysis, as well as immature protein-synthetic liver function. Mortality in such neonates reaches 75–90%.
Role in the Pathogenesis of DIC
In disseminated intravascular coagulation (DIC), there is a progressive increase in FDP levels. This is driven by significant platelet consumption and intensive secondary fibrinolysis triggered by activation of the kallikrein-kinin system.
- Stage of profound hypocoagulation (Stage III): typically develops 2–8 hours after the onset of the syndrome. Hyperplasminemia leads to the appearance of soluble fibrin-containing complexes and FDPs.
- The accumulation of FDPs enhances anticoagulant and antiplatelet effects. As a result, blood completely loses its clotting ability, leading to severe profuse bleeding, hemorrhage into unaffected tissues, and microangiopathic hemolytic anemia.
Clinical and Diagnostic Significance
- Laboratory Diagnostics: High levels of FDPs, including high- and low-molecular-weight fragments, are detected during intravascular coagulation. Elevated FDPs are also observed during active phases of rheumatic diseases, alongside elevated circulating immune complexes (CICs) and inflammatory markers.
- Obstetric Bleeding: In DIC, FDPs block receptors responsible for actomyosin formation, leading to massive hemorrhage.
- Internal Bleeding: Blood shed into the peritoneal cavity contains fibrinogen degradation products. Such blood is also characterized by decreased platelet and fibrinogen counts and high levels of free hemoglobin.
- Therapy: Plasmapheresis is the treatment of choice for detoxification and the elimination of FDPs, immune complexes, and bacterial toxins.