D-dimer

D-dimer (or D dimer) is a dimer that is a fibrin degradation product (FDP), a small protein fragment present in the blood after a blood clot is degraded by fibrinolysis. It is so named because it contains two D fragments of the fibrin protein joined by a cross-link, hence forming a protein dimer.^[1]

D-dimer concentration may be determined by a blood test to help diagnose thrombosis.^[2] Since its introduction in the 1990s, it has become an important test performed in people with suspected thrombotic disorders, such as venous thromboembolism.^[2][3] While a negative result practically rules out thrombosis, a positive result can indicate thrombosis but does not exclude other potential causes.^[3] Its main use, therefore, is to exclude thromboembolic disease where the probability is low.^[1][2]

D-dimer levels are used as a predictive biomarker for the blood disorder disseminated intravascular coagulation and in the coagulation disorders associated with COVID-19 infection.^[1][3] A four-fold increase in the protein is an indicator of poor prognosis in people hospitalized with COVID-19.^[1][3][4]