D-dimer, a small protein fragment, is found in blood only during the clotting process; otherwise it is normally undetected. When its presence in the blood rises, it suggests an increase in the formation and breakdown of clots. The article below will provide you with information on the normal plasma levels of this degradation product.
When a tissue or blood vessel is injured and begins to bleed, a process called hemostasis is initiated by the body that creates a blood clot, in order to stop the bleeding. Now, during this clotting process, thin protein threads called fibrin are formed, which go on to form a network by cross-linking, and this network along with platelets eventually stops the bleeding by forming a blood clot. It is during the clotting process that D-dimer, one of the protein fragments, is formed as a fibrin degradation product.
It is known as D-dimer because it contains two cross-linked D fragments of the protein fibrinogen. Although this molecule was discovered in the 1970s, its medical application came into being only in the 1990s.
» Indications and Significance
The concentration of D-dimer in the body is determined with the help of a blood test. These protein fragments, though not usually present in blood, are otherwise detected in blood plasma when the coagulation system has been activated. The presence of this protein is checked for, when there is a suspicion of
- A blood clot in the leg – deep venous thrombosis (DVT);
- A clot in the lung – pulmonary embolism (PE);
- Blood clot in an artery of another organ (like brain) – stroke;
- Clots in coronary arteries – myocardial infarction; or
- Numerous tiny blood clots – disseminated intravascular coagulation(arises due to various conditions like surgery, snakebites, liver disease, etc.), where the D-dimer level is very high.
The D-dimer test relies on the binding of a monoclonal antibody to one of the epitopes of the D-dimer fragment. The test helps to rule out the possibilities of any of the above indications; in fact, it is a rapid and noninvasive way to dismiss the possibility of abnormal or excessive clotting.
» D-dimer Range
| Normal Laboratory Values |
|
Normal or Negative Test
A normal or negative test result usually indicates that a suspected person may not be suffering from any acute condition that is causing unnatural blood clot formation or breakdown. This test is valid amongst people who are at a lesser risk of thrombosis.
This test eventually helps the doctor eliminate clotting as the reason for thrombotic diseases; in other words, the probability is reduced to less than 1% if the pretest probability is below 15-20%.
Positive Test
A positive test, on the other hand, tells the doctor that there may be a significant formation and breakdown of a thrombus which is indicated by abnormally elevated D-dimer levels. However, the location and cause of the blood clot may be undetectable. This calls for further testing to confirm the presence of a thrombus. Additional tests include an ultrasound of the leg veins, lung scintigraphy, or a CAT scan. In most cases though, anticoagulant therapy is initiated before the final results of the test are documented, so as to minimize the chances of damage to the body.
However, an elevated D-dimer level always does not point towards blood clotting. Here are the exceptions where fibrin formation and breakdown may take place due to which the D-dimer levels are unusually found to be high.
1. Pregnancy
2. A recent surgery
3. Heart disease
4. Liver disease (in this case, fibrin fails to clear normally)
» Variations
Most of the kits that are available for carrying out the D-dimer blood test claim to have a 93-95% sensitivity. Furthermore, these kits have about 50% specificity in diagnosing the presence of a thrombus (thrombotic diseases). However, there are some situations where the test results may not be 100% accurate. A false positive test result (in case of pregnancy, liver diseases, etc.) may show up, thus misleading someone regarding his D-dimer normal range. A false negative test, on the other hand, may be seen if the blood sample is taken too early after there is a thrombus formation, or if testing for the same has been delayed by many days.
The importance of this test lies in the fact that although a negative result nearly rules out thrombosis, a positive result can indicate thrombosis but does not rule out other potential causes of clotting. The main use of this test, therefore, is to exclude thromboembolic disease where the probability is low.
