Fig. 1

Mechanisms of production of: D-dimers, fibrin monomer, fibrinogen degradation products and fibrin degradation products. Fibrinogen is composed of two lateral regions “D” and one central region “E” connected by coiled coils: the formula is D-E-D. Fibrinopeptides A and B located at the N-termini of A-alpha and B-beta chains (× 2) on the E region are cleaved from fibrinogen by thrombin, resulting in the production of a fibrin monomer (FM). FMs are highly reactive and if locally formed and concentrated, quickly interact with one another by to form ​a two-stranded fibrin polymer. These polymers then aggregate laterally to make fibers (not shown). Activated factor XIII covalently crosslinks adjacent D regions (belonging to two fibrin monomers), which tightens the fibrin strand, increases clot stiffness, and makes it more resistant to degradation by plasmin. Other crosslinks also occur, not shown here for simplicity’s sake. The physical state is a gel - such polymerized structures are no longer soluble. During fibrinolysis, plasmin can cleave fibrin polymers between adjacent D and E regions, but cannot separate covalently linked D regions. This produces fibrin degradation products of different size, containing the ‘D-dimer’ motif, and when small enough are soluble. FM can escape in the fluid plasma phase, the more so if formed in a disseminated manner (systemic thrombin generation), and then quickly binds to fibrinogen molecules, or fibrinogen degradation products, bringing the polymerization process to an end; they hence remain soluble because they are small enough. These compounds are known as ‘soluble fibrin complexes’ (SFC). In the presence of hyperfibrinolysis (systemic, disseminated), PAI-1 (plasminogen activator inhibitor) and alpha2-antiplasmin can be overwhelmed, and uninhibited​ plasmin can diffuse in the plasma fluid phase; under those conditions, plasmin can also cleave fibrinogen molecules, resulting in fibrinogen degradation products production. ‘FDP’ may refer to both fibrinogen and fibrin degradation products