A blood sample drawn from a vein in your arm
D-dimer is one of the protein fragments produced when a blood clot gets dissolved in the body. It is normally undetectable or detectable at a very low level unless the body is forming and breaking down blood clots. Then, its level in the blood can significantly 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 significantly rise when there is significant formation and breakdown of fibrin 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 cut-off 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 diagnostic procedures are required (e.g., ultrasound, CT angiography).
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 lower 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 affect more than 300,000 people 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 test used?
D-dimer tests are used to help rule out the presence of an inappropriate blood clot (thrombus). Some of the conditions that the D-dimer test is used to help rule out include:
- Deep vein thrombosis (DVT)
- Pulmonary embolism (PE)
This test may be used to determine if further testing is necessary to help diagnose diseases and conditions that cause hypercoagulability, a tendency to clot inappropriately.
A D-dimer level may be used to help diagnose disseminated intravascular coagulation (DIC) and to monitor the effectiveness of DIC treatment.
When is it ordered?
D-dimer testing is often ordered when someone goes to the emergency room with symptoms of a serious condition (e.g., chest pain and difficulty in breathing).
A D-dimer test may be ordered when someone has symptoms of deep vein thrombosis, such as:
- Leg pain or tenderness, usually in one leg
- Leg swelling, edema
- Discoloration of the leg
It may be ordered when someone has symptoms of pulmonary embolism such as:
- Sudden shortness of breath, labored breathing
- Coughing, hemoptysis (blood present in sputum)
- Lung-related chest pain
- Rapid heart rate
D-dimer is especially useful when a healthcare practitioner thinks that something other than deep vein thrombosis or pulmonary embolism is causing the symptoms. It is a quick, non-invasive way to help rule out abnormal or excess clotting as the underlying cause. However, it should not be used when the probability of pulmonary embolism is high based on clinical assessment.
When a person has symptoms of disseminated intravascular coagulation (DIC), such as bleeding gums, nausea, vomiting, severe muscle and abdominal pain, seizures, and decreased urine output, a D-dimer test may be ordered, along with a PT, PTT, fibrinogen, and platelet count, to help diagnose the condition. D-dimer may also be ordered at intervals when someone is undergoing treatment for DIC to help monitor its progress.
What does the test result mean?
A normal or "negative" D-dimer result (D-dimer level is below a predetermined cut-off threshold) means that it is most likely that the person tested does not have an acute condition or disease causing abnormal clot formation and breakdown. Most health practitioners agree that a negative D-dimer is most valid and useful when the test is done for people who are considered to be at low to moderate risk for thrombosis. The test is used to help rule out clotting as the cause of symptoms.
A positive D-dimer result may indicate the presence of an abnormally high level of fibrin degradation products. It indicates that there may be significant blood clot (thrombus) formation and breakdown in the body, but it does not tell the location or cause. For example, it may be due to a venous thromboembolism (VTE) or disseminated intravascular coagulation (DIC). Typically, the D-dimer level is very elevated in DIC.
However, an elevated D-dimer does not always indicate the presence of a clot because a number of other factors can cause an increased level. Elevated levels may be seen in conditions in which fibrin is formed and then broken down, such as recent surgery, trauma, infection, heart attack, and some cancers or conditions in which fibrin is not cleared normally, such as liver disease. Therefore, D-dimer is typically not used to rule out VTE in hospitalized patients (inpatient setting).
Fibrin is also formed and broken down during pregnancy, so that may result in an elevated D-dimer level. However, if DIC is suspected in a woman who is pregnant or is in the immediate postpartum period, then the D-dimer test may be used, along with a PT, PTT, fibrinogen, and platelet count to help diagnose her condition. If the woman has DIC, her D-dimer level will be very elevated.
D-dimer is recommended as an adjunct test. Since D-dimer is a sensitive test but has a poor specificity, it should only be used to rule out deep vein thrombosis (DVT), not to confirm a diagnosis. It should not be used for pulmonary embolism when the clinical probability of that condition is high. Both increased and normal D-dimer levels may require follow-up and can lead to further diagnostic testing. People with positive D-dimer tests and those with moderate to high risk for DVT require further study with diagnostic imaging (e.g., CT angiography).
When used to monitor DIC treatment, decreasing levels indicate that treatment is effective while increasing levels may indicate that treatment is not working.
Is there anything else I should know?
D-dimer concentrations rise in the elderly, and false positives may be seen with high levels of rheumatoid factor, a protein seen in people with rheumatoid arthritis. D-dimer testing is not well studied in children.
There are several different methods of testing for D-dimer. The currently available D-dimer tests that yield quantitative results are typically done in a hospital lab. Since different tests are used in different units, the results of one cannot be extrapolated to another.
What are some common risk factors for inappropriate blood clotting?
Some risk factors include:
- Major surgery or trauma
- Hospitalization or living in a nursing home
- Prolonged immobility—this can include long trips by plane, car, etc. or prolonged bed rest
- Use of birth control or hormone replacement therapy
- Broken bone, cast
- Pregnancy or recent childbirth
- Antiphospholipid syndrome
- Certain cancers
- Inherited clotting disorder such as factor V Leiden mutation
- History of prior venous thromboembolism (VTE)
What other procedures might my healthcare practitioner order if my D-dimer is positive?
In an emergency room setting, if you are found to be at low to intermediate risk for thrombosis and/or venous thromboembolism and when you have a positive D-dimer test, your healthcare practitioner will likely order a non-invasive scanning procedure, such as a venous ultrasound, multi-detector helical computed axial tomography (CT), direct pulmonary angiography, or ventilation-perfusion (V/Q) scan. For more on these, see RadiologyInfo.org: Blood Clots.