D-dimer

D-dimer is a sensitive (95%), but not specific, blood test for VTE. This makes it helpful in excluding VTE where there is a low suspicion. It is almost always raised if there is a DVT; however other conditions can also cause a raised d-dimer:

  • Surgery
  • Trauma
  • Pneumonia
  • Malignancy
  • Heart failure
  • Pregnancy (It is never worth doing a D-dimer in pregnancy)

D-dimer is a specific protein fragment that is produced when a blood clot dissolves in the body. It is a small protein piece present in the blood after a blood clot is degraded by fibrinolysis. D-dimer testing is an important diagnostic tool, particularly in assessing the presence of an acute blood clot. Here are key points about D-dimer:

  1. Role in Blood Clotting: When a blood clot forms in response to an injury or other factors, it is eventually broken down by the body in a process called fibrinolysis. During this process, fibrin, a protein that helps form the clot, is degraded, and D-dimer is released into the bloodstream.
  2. D-dimer Test: This blood test measures the amount of D-dimer present in the blood. The test is highly sensitive but not very specific. This means that while a negative D-dimer test can effectively rule out the presence of an active blood clot (like deep vein thrombosis or pulmonary embolism), a positive result does not necessarily confirm one.
  3. Uses of D-dimer Testing:
    1. Deep Vein Thrombosis (DVT) and Pulmonary Embolism (PE): It is commonly used in the diagnosis of these conditions. A low or normal D-dimer level suggests that it is unlikely that symptoms are due to a blood clot.
    2. Disseminated Intravascular Coagulation (DIC): Elevated levels can also be seen in DIC, a condition where small blood clots develop throughout the bloodstream.
    3. Other Conditions: High levels of D-dimer can also be seen in pregnancy, liver disease, inflammation, cancer, and after surgery, which is why it’s not specific for blood clots.
  4. Limitations of D-dimer Testing: Due to its lack of specificity, a positive D-dimer test needs to be followed by other imaging tests (like ultrasound for DVT or CT scan for PE) to confirm the diagnosis of a blood clot. The test is more useful in ruling out a clot than confirming one.
  5. Age-Adjusted D-dimer: Recognizing that D-dimer levels increase with age, some protocols adjust the cutoff values based on the patient’s age, which can improve the test’s diagnostic accuracy in older patients.
  6. Clinical Decision Making: D-dimer tests are often used in conjunction with clinical assessment and other diagnostic tools. For instance, in patients with a low clinical probability of DVT or PE, a negative D-dimer test can reliably exclude these diagnoses.

In summary, the D-dimer test is a valuable tool in the initial evaluation of conditions like DVT and PE. It is best used in the context of the patient’s symptoms and other diagnostic findings due to its high sensitivity but low specificity.