When you have symptoms of a disease or condition that causes acute and/or chronic inappropriate blood clot formation such as: DVT (Deep Vein Thrombosis), PE (Pulmonary Embolism), or DIC (Disseminated Intravascular Coagulation), and to monitor the progress and treatment of DIC and other thrombotic conditions.
A blood sample taken from a vein in your arm or a fingerprick sample for some point of care tests
When a vein or artery is injured and begins to leak blood, a sequence of clotting steps and factors (called the coagulation cascade) is activated by the body to limit bleeding and create a blood clot to plug the hole. During this process, threads of a protein called fibrin are produced. These threads are cross-linked (glued together by a protein called thrombin) to form a fibrin net that catches platelets and helps hold the forming blood clot together at the site of the injury.
Once the area has had time to heal, the body uses a protein called plasmin to break the clot (thrombus) into small pieces so that it can be removed. The fragments of the disintegrating clot are called fibrin degradation products (FDP). One of the FDPs produced is D-dimer, which consists of variously sized pieces of cross-linked fibrin. D-dimer is normally undetectable in the blood and is produced only after a clot has formed and is in the process of being broken down.
The main use of D-Dimer is in ruling-out venous blood clots as a cause for symptoms in the legs or lungs. If your D-dimer result is negative it is extremely unlikely you have a significant venous blood clot. If a D-dimer is positive this does not mean that you have a blood clot, as it is raised in many conditions. D-dimer is therefore not useful for ruling-in or confirming a diagnosis of blood clots.
How is it used?
D-dimer tests are requested, along with other laboratory tests and imaging scans, to help exclude, diagnose, and monitor diseases and conditions that cause hypercoagulability, a tendency to clot inappropriately. One of the most common of these conditions is DVT (Deep Vein Thrombosis) which involves clot formation in the deep veins of 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, where the clot can lodge in the lungs (known as Pulmonary Embolism or PE).
If you have symptoms of a DVT or PE a doctor will use something called a ‘Wells score’ to decide if a blood clot is likely or unlikely. This will help your doctor to decide whether you need a d-dimer test.
Measurements of D-dimer may also be requested, along with other tests, to help diagnose DIC (Disseminated Intravascular Coagulation). DIC is a complex rapidly developing disease that can arise from a variety of situations including: some surgical procedures, burns, infections, cancer, liver disease, poisonous snake bites, and postpartum (after the delivery of a baby). With DIC, clotting factors are activated and then used up throughout the body. This creates numerous minute or larger blood clots and at the same time leaves the patient vulnerable to excessive bleeding. Steps may be needed to support the patient with anticoagulants or blood product replacement, while the underlying problem is treated. D-dimer levels may be used to monitor the effectiveness of DIC treatment.
When is it requested?
D-dimer may be requested when
- a patient has symptoms of DVT, such as leg pain, tenderness, oedema (swelling), discolouration
- symptoms of PE, such as breathlessness, cough, and lung-related chest pain. D-dimer is especially useful when the doctor thinks that something other than DVT or PE is causing the symptoms. It is a quick, non-invasive way for the doctor to help exclude abnormal or excess clotting.
- a patient has symptoms of DIC, such as mucosal bleeding (from the mouth, nose, bladder or bowels), bruising, nausea, vomiting, severe muscle and abdominal pain, seizures and oliguria (decreased urine output), a D-dimer test may be requested, with PT, aPTT, fibrinogen and platelet count to help diagnose the condition. D-dimer may also be used at when a patient is undergoing treatment for DIC to help monitor its progress.
What does the test result mean?
A positive D-dimer indicates the presence of an abnormally high level of cross-linked fibrin degradation products in your body. It tells your doctor that there has been significant clot (thrombus) formation and breakdown in the body, but it does not identify the location or cause. An elevated D-dimer may be due to a VTE or DIC but it may also be due to a recent surgery, or trauma, infection, liver or kidney disease, cancers, in normal pregnancy but also some diseases of pregnancy such as eclampsia.
A normal D-dimer test means that it is unlikely you have an acute blood clot or disease causing abnormal clot formation and breakdown. Most doctors agree that a negative D-dimer is most valid and useful when the test is done on patients that are considered to be low-risk. The test is used to help exclude a clot as the cause of symptoms.
D-dimer is recommended as an ‘additional test’. It should not be the only test used to diagnose a disease or condition. Both increased and normal D-dimer levels may require follow-up and can lead to further testing.
Is there anything else I should know?
Anticoagulant therapy can cause a false negative D-dimer. There are several different methods of testing for D-dimer. Most of the D-dimer tests that yield quantitative results are done in a hospital lab, while those that yield qualitative results are performed at the patient’s bedside.
D-dimer concentrations may rise in the elderly, and false positives may be seen with high levels of rheumatoid factor (a protein seen in patients with rheumatoid arthritis). Substances such as lipaemia (a large amount of fats in the blood that can be caused by consuming a greasy meal prior to testing), and raised bilirubin can also cause false positives as can haemolysis (rupturing of red blood cells) caused by improper collection and handling.
Is there a home test for D-dimer?
What other procedures might my doctor request if my D-dimer is positive?
What are some common risk factors for inappropriate blood clotting?
Some risk factors include:
- Being overweight
- Major illness
- Surgery or trauma
- Hospitalisation or prolonged bed rest
- long trips by plane, car, etc
- broken bones, casts
- Use of birth control or hormone replacement therapy
- Pregnancy or recent childbirth
- Abnormally sticky blood
- Inherited clotting disorder such as factor V Leiden mutation
- Non-inherited disorders such as antiphospholipid syndrome
Elsewhere On The Web
Diagnosis of deep vein thrombosis in symptomatic outpatients and the potential for clinical assessment and D-dimer assays to reduce the need for diagnostic imaging
Guideline on the investigation, management and prevention of venous thrombosis in children
Guidelines on travel-related venous thrombosis
Guidelines for the investigation and management of disseminated intravascular coagulation