D-Dimer

From: The Wikipedia

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.[citation needed]

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:[3]
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.[4]
[edit] Test properties
Various kits have a 93-95% sensitivity and about 50% specificity in the diagnosis of thrombotic disease.[5]
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.