D-dimer is a blood test performed in the medical laboratory to diagnose thrombosis. Since its introduction in the 1990s, it has become an important test performed in patients suspected of thrombotic disorders. While a negative result practically rules out thrombosis, a positive result can indicate thrombosis but also has other potential causes. Its main use, therefore, in to exclude thromboembolic disease where the probability is low.
D-dimer testing is of clinical use when there is a suspicion of deep venous thrombosis (DVT) or pulmonary embolism (PE). In patients suspected of disseminated intravascular coagulation (DIC), D-dimers may aid in the diagnosis.
For DVT and PE, there are various scoring systems that are used to determine the probability of these diseases; the best-known were introduced by Wells et al (2003).
- In a very high score, a D-dimer will make little difference, and anticoagulant therapy will be initiated regardless of test results.
- In a moderate or low score:
- A negative D-dimer test will virtually rule out thromboembolism.
- If the D-dimer reads high, then further testing (ultrasound of the leg veins or lung scintigraphy or CT scanning) is required to confirm the presence of thrombus before anticoagulant therapy can be continued.
Most sampling kits have 0-300 ng/ml as normal range. Values exceeding 250, 300 or 500 ng/ml (different for various kits) are considered positive.
Types of assays
- ELISA (e.g. SimpliRED®, Vidas®)
- Latex turbidimetric assay (automated immunoassay, e.g. Roche Tina-quant®, MDA D-dimer®)
- Enhanced microlatex
- Latex-enhanced photometric
Principles of D-dimer testing
Fibrin degradation products (FDPs) are formed whenever fibrin is broken down by enzymes (e.g. plasminogen). 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 stabilised 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 natural fibrinolytic enzyme that organises 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. The first patented MoAb of the kind was D Dimer-3B6/22, although others have been developed.
Sensitivity and specificity
Various kits have a 93-95% sensitivity and about 50% specificity in the diagnosis of thrombotic disease.
D-dimer testing was originally developed in the diagnosis of disseminated intravascular coagulation. In the 1990s, they turned out to be useful in thromboembolic disorders.
- Schrecengost JE, LeGallo RD, Boyd JC, Moons KG, Gonias SL, Rose CE Jr, Bruns DE. Comparison of diagnostic accuracies in outpatients and hospitalized patients of D-dimer testing for the evaluation of suspected pulmonary embolism. Clin Chem 2003;49:1483-90. PMID 12928229.
- Wells PS, Anderson DR, Rodger M, Forgie M, Kearon C, Dreyer J, Kovacs G, Mitchell M, Lewandowski B, Kovacs MJ. Evaluation of D-dimer in the diagnosis of suspected deep-vein thrombosis. N Engl J Med 2003;349:1227-35. PMID 14507948.
- D-dimer reference centre (http://www.agen.com.au/d-dimer/default.htm) (site of a manufacturer of D-dimer sampling kits)