D-dimer

D-dimer (or D dimer) is a

D-dimers are not normally present in human blood plasma, except when the coagulation system has been activated, for instance, because of the presence of thrombosis or disseminated intravascular coagulation. The D-dimer assay depends on the binding of a monoclonal antibody to a particular epitope on the D-dimer fragment. Several detection kits are commercially available; all of them rely on a different monoclonal antibody against D-dimer. For some of these, the area of the D-dimer to which the antibody binds is known. The binding of the antibody is then measured quantitatively by one of various laboratory methods.^[1]

Indications

D-dimer testing is of clinical use when there is a suspicion of

For DVT and PE, there are possible various scoring systems that are used to determine the a priori clinical probability of these diseases; the best-known is the Wells score.^[5]

• For a 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:^{[citation needed]}
  • A negative D-dimer test will virtually rule out thromboembolism:^[5] 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 the 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%. Chest computed tomography (CT angiography) should not be used to evaluate pulmonary embolism for persons with negative results of a D-dimer assay.^[9] A low pretest probability is also valuable in ruling out PE.^[10]
  • If the D-dimer reads high, then further testing (
    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.

The following are reference ranges for D-dimer:^[16]

D-dimer increases with age. It has therefore been suggested to use a cutoff equal to patient’s age in years × 10 µg/L (or x 0.056 nmol/L) for patients aged over 50 years for the suspicion of venous thromboembolism (VTE), as it decreases the

An alternative measurement of D-dimer is in fibrinogen equivalent units (FEU). The molecular weight of the fibrinogen molecule is about twice the size of the D-dimer molecule, and therefore 1.0 mcg/mL FEU is equivalent to 0.5 mcg/mL of d-dimer.[19]

Thrombotic disease

Various kits have a 93 to 95% sensitivity (true positive rate). For hospitalized patients, one study found the specificity to be about 50% (related to false positive rate) in the diagnosis of thrombotic disease.^[20]

• trauma, pregnancy, recent surgery as well as advanced age.^[21]
• 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. The anti-coagulation medications dabigatran and rivaroxaban decrease D-dimer levels but do not interfere with the D-dimer assay.^[22]
• False values may be obtained if the specimen collection tube is not sufficiently filled (false low value if underfilled and false high value if overfilled). This is due to the dilutional effect of the anticoagulant (the blood must be collected in a 9:1 blood to anticoagulant ratio).
• Likelihood ratios are derived from sensitivity and specificity to adjust pretest probability.

In interpretation of the D-dimer, for patients over age 50, a value of (patient's age) × 10 μg/L may be abnormal.[23]^[24]

History

D-dimer was originally described in the 1970s and found its diagnostic application in the 1990s.^[1][5]

References