D-dimer

Overview
D-dimer is a fibrin degradation product, a small protein fragment present in the blood after a blood clot is degraded by fibrinolysis.

D-dimer concentration may be determined in a tested to help 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 does not rule out other potential aetiologies. Its main use, therefore, is to exclude thromboembolic disease where the probability is low.

Indications
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 a priori clinical probability of these diseases; the best-known were introduced by Wells et al (2003).
 * For a very high score, or pretest probability, a D-dimer will make little difference and anticoagulant therapy will be initiated regardless of test results, and additional testing for DVT or pulmonary embolism may be performed.
 * For a moderate or low score, or pretest probability:
 * A negative D-dimer test will virtually rule out thromboembolism: the degree to which the d-dimer reduces the probability of thrombotic disease is dependent on the test properties of the specific test used in your clinical setting: most available d-dimer tests with a negative result will reduce the probability of thromboembolic disease to less than 1% if the pretest probability is less than 15-20%
 * 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. Anticoagulant therapy may be started at this point or withheld until further tests confirm the diagnosis, depending on the clinical situation.

In some hospitals, they are measured by laboratories after a form is completed showing the probability score and only if the probability score is low or intermediate. This would reduce the need for unnecessary tests in those who are high-probability.

Reference range
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. Vidas)
 * Latex turbidimetric assay (automated immunoassay, e.g. Roche Tina-quant, MDA D-dimer)
 * Enhanced microlatex
 * Latex-enhanced photometric
 * Whole Blood Agglutination (e.g. SimpliRED)
 * Rapid Lateral Flow (e.g. Clearview Simplify)

Principles
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. The first patented MoAb of the kind was D Dimer-3B6/22, although others have been developed.



Test Properties
Various kits have a 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.
 * Likelihood ratios are derived from sensitivity and specificity to adjust pretest probability.

History
D-dimer testing was originally developed in the diagnosis of disseminated intravascular coagulation. In the 1990s, they turned out to be useful in diagnosing thromboembolic process.