D-dimer

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).  

Most clots travel in the body's veins, but clotting can also sometimes occur in the oxygen-carrying arteries. The combination of these two parts of thrombosis is sometimes referred to as venous thromboembolism (VTE). If a blood clot blocks the flow of blood to a vital organ, such as a kidney, the brain, or the heart, it may cause irreversible damage (infarction) and can lead to organ failure. D-dimer levels are generally requested to make sure that they are not elevated. They are used to help exclude DVT and PE.

Measurements of D-dimer may also be requested, along with other tests, to help diagnose DIC (Disseminated Intravascular Coagulation). DIC is a complex acute condition 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 are taken to support the patient, while the underlying problem is addressed, and the underlying condition resolved. D-dimer levels may be used to monitor the effectiveness of DIC treatment.

D-dimer may be requested when a patient has symptoms of DVT, such as leg pain, tenderness, oedema (swelling), discolouration,; or 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.

When 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.

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, pregnancy and diseases of pregnancy such as eclampsia, heart disease, and some cancers.

A normal D-dimer test means that it is most 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.

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.