What are D-dimers?

By William Aird

  • Degradation product of cross-linked fibrin.
  • The smallest circulating fibrin degradation product that is specific for fibrinolysis (as distinct from fibrinogenolysis).
  • Contains two cross-linked D fragments of the fibrin protein, hence its name.
  • High levels in patients with disseminated intravascular coagulation (DIC), but high levels can also be found in patients with venous thromboembolism, recent surgery, or inflammatory conditions.
Fibrinogen is cleaved by thrombin to generate fibrin. Fibrin is then crosslinked by FXIIIa, resulting in an insoluble gel. Crosslinked fibrin is degraded by plasmin. This process, termed fibrinolysis yields soluble fibrin degradation products (FDPs). Fibrinogen may also be cleaved by plasmid to generate FDPs (fibrinogenolysis). FDPs derived from fibrin and fibrinogen are identical with the exception of D-dimer, which requires the action of plasmin on crosslinked fibrin.
A more granular look at formation of FDPs and D-dimers from fibrinogen and fibrin. Fibrinogen is a dumbbell-shaped molecule with 2 D domains (red) flanking a single E domain (yellow). The sequential action of thrombin and FXIII results in the formation of crosslinked fibrin, with the crosslinks occurring between D domains on the longitudinal axis. Plasmin degrades fibrinogen and fibrin into fibrin degradation products (FDPs). Note that the D-dimer is unique to the breakdown of crosslinked fibrin. Other FDPs, namely D and E fragments are generated from plasmin-mediated degradation of both fibrinogen and fibrin. FDP assays pick up all FDP fragments (from fibrinogen and fibrin breakdown), whereas the D-dimer assay measures D-dimer alone (from fibrin breakdown).