A by-product of the degradation of blood clots, specifically, of the fibrin within a clot. The presence of an elevated level of D-dimer in the setting of a suspected pulmonary embolus or deep venous thrombosis is justification for further testing. The absence of D-dimer in plasma is used to help exclude a diagnosis of deep venous thrombosis or pulmonary embolism when the test is performed with an enzyme-linked immunosorbent assay (ELISA).