D-dimer tests

D-dimer testing is of clinical use when there is a suspicion of deep venous thrombosis (DVT) or pulmonary embolism (PE).
While a negative result practically rules out thrombosis, a positive result can indicate thrombosis but does not rule out other potential causes. Its main use, therefore, is to exclude thromboembolic disease where the probability is low. – some hospitals wont even carry out the test unless the probability is either low or moderate of thromboembolic disease.
Fibrin degradation products (FDPs) are formed whenever fibrin is broken down by enzymes (e.g., plasmin). Determining FDPs is not considered useful, as this does not indicate whether the fibrin is part of a blood clot (or being generated as part of inflammation).
D-dimers are unique in that they are the breakdown products of a fibrin mesh that has been stabilized by Factor XIII. This factor crosslinks the E-element to two D-elements. This is the final step in the generation of a thrombus.
Plasmin is a fibrinolytic enzyme that organizes clots and breaks down the fibrin mesh. It cannot, however, break down the bonds between one E and two D units. The protein fragment thus left over is a D-dimer.

D-dimer assays rely on monoclonal antibodies to bind to this specific protein fragment. Various kits have 93-95% sensitivity and about 50% specificity in the diagnosis of thrombotic disease.
• False positive readings can be due to various causes: liver disease, high rheumatoid factor, inflammation, malignancy, trauma, pregnancy, recent surgery as well as advanced age
• False negative readings can occur if the sample is taken either too early after thrombus formation or if testing is delayed for several days. Additionally, the presence of anti-coagulation can render the test negative because it prevents thrombus extension.