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The Test Sample
What is being tested?
D-dimer is one of the protein fragments produced when a blood clot dissolves in the body. It is normally undetectable unless the body is forming and breaking down blood clots. Then, its level in the blood can rise. This test detects D-dimer in the blood.
When a blood vessel or tissue is injured and begins to bleed, a process called hemostasis is initiated by the body to create a blood clot to limit and eventually stop the bleeding. This process produces threads of a protein called fibrin, which crosslink together to form a fibrin net. That net, together with platelets, helps hold the forming blood clot in place at the site of the injury until it heals.
Once the area has had time to heal and the clot is no longer needed, the body uses an enzyme called plasmin to break the clot (thrombus) into small pieces so that it can be removed. The fragments of the disintegrating fibrin in the clot are called fibrin degradation products (FDP), which consist of variously sized pieces of crosslinked fibrin. One of the final fibrin degradation products produced is D-dimer, which can be measured in a blood sample when present. The level of D-dimer in the blood can rise when there is significant formation and breakdown of blood clots in the body.
For a person who is at low or intermediate risk for blood clotting (thrombosis) and/or thrombotic embolism, the strength of the D-dimer test is that it can be used in a hospital emergency room setting to determine the likelihood of a clot's presence. A negative D-dimer test (D-dimer level is below a predetermined threshold) indicates that it is highly unlikely that a thrombus is present. However, a positive D-dimer test cannot predict whether or not a clot is present. It indicates that further testing is required.
There are several factors and conditions associated with inappropriate blood clot formation. One of the most common is deep vein thrombosis (DVT), which involves clot formation in veins deep within the body, most frequently in the legs. These clots may grow very large and block blood flow in the legs, causing swelling, pain, and tissue damage. It is possible for a piece of the clot to break off and travel to other parts of the body. This "embolus" can lodge in the lungs, causing a pulmonary embolus or embolism (PE). Pulmonary embolisms from DVT cause as many as 300,000 deaths in the U.S. each year.
While clots most commonly form in the veins of the legs, they may also form in other areas as well. Measurements of D-dimer can be used to help detect clots in any of these sites. For example, clots in coronary arteries are the cause of myocardial infarction (heart attacks). Clots may form on the lining of the heart or its valves, particularly when the heart is beating irregularly (atrial fibrillation) or when the valves are damaged. Clots can also form in large arteries as a result of narrowing and damage from atherosclerosis. Pieces of such clots may break off and cause an embolus that blocks an artery in another organ, such as the brain (causing a stroke) or the kidneys.
Measurements of D-dimer may also be ordered, along with other tests, to help diagnose disseminated intravascular coagulation (DIC). DIC is a condition in which clotting factors are activated and then used up throughout the body. This creates numerous tiny blood clots and at the same time leaves the affected person vulnerable to excessive bleeding. It is a complex, sometimes life-threatening condition that can arise from a variety of situations, including some surgical procedures, sepsis, poisonous snake bites, liver disease, and after childbirth. Steps are taken to support the affected person while the underlying condition resolves. The D-dimer level will typically be very elevated in DIC.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.