Fibrin Degradation Fragment, more commonly referred to as D Dimer, is a final fibrin degradation product produced when a clot is dissolved, marking the end of the clotting cascade. Normally undetectable in the blood, elevated levels of D Dimer can be indicative of pathological blood coagulation, as levels will not typically be detectable unless the body’s coagulation system is active.
Human coagulation systems work via a process known as hemostasis, in which the body attempts to regulate the creation and degradation of blood clots. Activation of the clotting cascade occurs after an injury, where blood vessels have been damaged thus causing their contents to leak. Initially, the vessel wall contracts to minimise blood loss, followed by the recruitment of platelets to the site. Via adherence to both other platelets and exposed collagen, these platelets aggregate to form a plug in the vessel. Platelets are also responsible for the recruitment of numerous other proteins essential to clotting.
Many of the coagulation factors necessary to form blood clots are already present in the bloodstream as zymogens, activated by signalled release of factors by platelets after injury. A key protein is thrombin, produced by the proteolytic cleavage of its precursor, prothrombin. Thrombin is a serine protease whose function is to convert fibrinogen into fibrin whilst also activating other important factors. Fibrin is a fibrous, non-globular protein which forms a mesh at the wound site, with its crosslinking facilitated by Factor XIII.
After the clot has fulfilled its function, it must be dissolved in order to restore normal blood flow through the vessel. Plasmin in its zymogen form, plasminogen, is present in the circulation; in contact with a fibrin clot, numerous activators convert it into the active plasmin form, which is a serine protease capable of degrading fibrin via fibrinolysis. One of the smallest products of endogenous fibrin degradation is D Dimer, a 180kDa dimer formed of fragments of the three polypeptide chains found in fibrin; alpha, beta and gamma, held together by disulphide bonds.
Due to the nature of its formation, an increase in D Dimer in the blood can be indicative of recent clotting and therefore can be used to identify pathogenic clotting, whereby there is an imbalance in hemostasis. Quantitative tests for D Dimer hold their importance in their negative results: a negative D Dimer test can be used to rule out conditions such as pulmonary embolism, deep vein thrombosis, stroke and disseminated intravascular coagulation. Elevated D Dimer levels can be indicative of one of these conditions, however, positive tests are not conclusive and require further medical investigation, as results do not indicate the location or cause of high levels of D Dimer. It is also difficult to classify normal levels of D Dimer, hence the emphasis placed on negative testing.
With D Dimer being heavily prevalent in research into clotting and its related disorders, such as thrombosis, highly accurate and sensitive assay methods are constantly required to provide precise concentrations within samples. For efficiency, the preferred method to do so is via the use of a D Dimer ELISA Kit, specifically a competitive assay. Comprised of a 96 well plate pre-coated with antibody specific for D Dimer, a competitive inhibition reaction between the labelled and unlabelled dimer is performed, with the unbound conjugates washed away. After further incubation and substrate addition, only wells containing D Dimer will yield a blue product which changes to yellow following the addition of a stop solution. O.D absorbance can be measured, which is inversely proportional to the amount of D Dimer bound to the plate. Spectrophotometric absorbance values can be used to calculate D Dimer concentration in the sample.
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