D-Dimer Blood Test

The D-dimer blood test is a very useful laboratory test that helps doctors rule out pulmonary embolism or deep vein thrombosis (DVT) in a patient. This is not a routine blood test and is performed only during emergencies or in cases where the patient is in pain.

When a vein or artery is injured, it may leak blood into the body. In turn, the body activates a sequence of clotting steps and factors called the coagulation cascade to control and stop the bleeding. When the process of clotting is carried out, threads of proteins are produced called fibrin.

These threads are cross linked together forming a net that can catch the platelets and hold the blood clot together, near the site of injury. Once the area of injury heals, the body releases a protein called plasmin. This protein is used to break the thrombus (clot) into small pieces. This helps in removing the thrombus formed within the body.

The disintegrating fibrin fragments are called fibrin degradation products (FDP). These FDP also contain D-dimer that is formed by two cross linked fragments of the fibrinogen protein. D-dimer is undetectable under normal circumstances and is produced only after a clot is formed and is always in the process of breaking down.

The level of D-dimer in the blood, helps the doctor determine if pulmonary embolism or deep vein thrombosis has occurred in a patient. However, a positive test does not always indicates clots or thrombosis. It may also be elevated in case of infection or trauma to surrounding tissues.

The test is conducted by drawing a blood sample from the patient's vein by a phlebotomist. The vein of the arm is the obvious choice for drawing a sample.

However, in some cases when the vein in the arm is not usable, other veins located in the body can be used. The sample drawn from the arm is then sent to the lab for further processing. The result helps the doctor determine, if further tests are needed or not.

The test is only ordered if the doctor suspects deep venous thrombosis or pulmonary embolism. D-dimers help in the diagnosis of disseminated intravascular coagulation (DIC). The test's normal range indicates a negative test.

When the D-dimer quantity in the blood is below a set cut off value, it means that there is not much clot turnover in the patient's body. Thus, normal range rules out thrombosis. A D-dimer blood test with positive result may call for further tests like ultrasound of the leg veins or CT scan or lung scintigraphy.

These tests will help determine presence of thrombus in a patient. The doctor may begin with anticoagulant therapy or wait for further results, depending on the patient's case.

In case of false positive readings, it may indicate other causes like liver diseases, inflammation, malignancy, trauma, pregnancy, high rheumatoid factor, or even advanced age. A false negative test result may be seen when the sample is taken too early after thrombosis formation, or if the result was delayed for several hours.

This is not a routine test and is carried out only if the doctor suspects blood clots or thrombosis. The quantitative D-dimer blood tests are performed in labs and qualitative tests can be performed even at the patient's bedside. I hope this article has helped you understand this diagnostic test to rule out thrombosis or disseminated intravascular coagulation.

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