Editing D-dimer (section)

==Principles==
[[Image:D-dimer.png|right|framed|'''D-dimer formation.''' Shown are ''[[fibrinogen]]'', with its one ''E'' domain and two ''D'' domains, acted upon in cascade, by the following [[enzyme]]s: ''[[Thrombin]]'', to create a ''mesh of fibrin'' protofibrils; [[Factor XIII]] to ''crosslink the fibrin mesh'' (linking protofibril D domains), the scaffold for [[blood clot|clot]] formation; ''[[Plasmin]]'', whose action in [[fibrinolysis]] produces [[fibrin degradation products]] (''FDPs''), the smallest of which are ''D-dimers'', protein fragments with one E and two crosslinked D domains from an original fibrinogen.<ref name="Asakura" /><ref name=wells/>]]
[[Coagulation]], the formation of a blood clot or [[thrombus]], occurs when the proteins of the [[coagulation cascade]] are activated, either by contact with a damaged blood vessel wall and exposure to collagen in the tissue space (intrinsic pathway) or by activation of [[factor VII]] by [[tissue factor|tissue activating factors]] (extrinsic pathway). Both pathways lead to the generation of [[thrombin]], an enzyme that turns the soluble blood protein [[fibrin]]ogen into fibrin, which aggregates into protofibrils. Another thrombin-generated enzyme, [[factor XIII]], then crosslinks the fibrin protofibrils at the D fragment site, leading to the formation of an insoluble gel that serves as a scaffold for blood clot formation.<ref name="Asakura" />

The circulating enzyme [[plasmin]], the main enzyme of [[fibrinolysis]], cleaves the fibrin gel in a number of places. The resultant fragments, "high molecular weight polymers", are digested several times more by plasmin to lead to intermediate and then to small polymers ([[fibrin degradation product]]s or FDPs). The cross-link between two D fragments remains intact, however, and these are exposed on the surface when the fibrin fragments are sufficiently digested. The structure of D-dimer is either a 180 [[Unified atomic mass unit|kDa]]<ref name="KoganMukharyamova2016">{{cite journal | vauthors = Kogan AE, Mukharyamova KS, Bereznikova AV, Filatov VL, Koshkina EV, Bloshchitsyna MN, Katrukha AG | title = Monoclonal antibodies with equal specificity to D-dimer and high-molecular-weight fibrin degradation products | journal = Blood Coagulation & Fibrinolysis | volume = 27 | issue = 5 | pages = 542–50 | date = July 2016 | pmid = 26656897 | pmc = 4935535 | doi = 10.1097/MBC.0000000000000453 }}</ref> or 195 kDa<ref name="OlsonCunningham2013">{{cite journal | vauthors = Olson JD, Cunningham MT, Higgins RA, Eby CS, Brandt JT | title = D-dimer: simple test, tough problems | journal = Archives of Pathology & Laboratory Medicine | volume = 137 | issue = 8 | pages = 1030–8 | date = August 2013 | pmid = 23899057 | doi = 10.5858/arpa.2012-0296-CP }}</ref> molecule of two D domains, or a 340 kDa<ref name="OlsonCunningham2013"/> molecule of two D domains and one E domain of the original fibrinogen molecule.<ref name="Asakura" /> The [[biological half-life|half-life]] of D-dimer in blood is approximately 6 to 8&nbsp;hours.<ref name="pmid20213258">{{cite journal | vauthors = Lippi G, Cervellin G, Franchini M, Favaloro EJ | title = Biochemical markers for the diagnosis of venous thromboembolism: the past, present and future | journal = J Thromb Thrombolysis | volume = 30 | issue = 4 | pages = 459–71 | date = November 2010 | pmid = 20213258 | doi = 10.1007/s11239-010-0460-x | s2cid = 23806848 | url = }}</ref>

D-dimers are not normally present in human blood plasma, except when the coagulation system has been activated, for instance, because of the presence of [[thrombosis]] or [[disseminated intravascular coagulation]]. The D-dimer assay depends on the binding of a [[monoclonal antibody]] to a particular [[epitope]] on the D-dimer fragment. Several detection kits are commercially available; all of them rely on a different monoclonal antibody against D-dimer. For some of these, the area of the D-dimer to which the antibody binds is known. The binding of the antibody is then measured quantitatively by one of various laboratory methods.<ref name="Asakura" />