The D-dimer test measures a specific protein fragment released into the bloodstream when a blood clot is actively being broken down. Since the body is constantly forming and dissolving small clots, a low level of this fragment is expected at all times. The presence of elevated D-dimer levels suggests that significant clot formation and subsequent breakdown, known as fibrinolysis, have occurred somewhere in the body.
What D-dimer Measures in the Blood
D-dimer is a fibrin degradation product (FDP). The process begins with coagulation, converting soluble fibrinogen into insoluble fibrin strands to form a primary clot. These strands are then cross-linked by Factor XIII, creating a stable net. The body activates the fibrinolytic system, using the enzyme plasmin to dissolve this stable fibrin mesh. When plasmin breaks down the cross-linked fibrin, it releases D-dimer, confirming that both clot formation and breakdown have taken place. Measuring this molecule provides an indirect marker of active coagulation and fibrinolysis occurring within the vascular system.
Understanding Normal Reference Ranges
For most healthy adults, the standard threshold for a normal D-dimer result is below 500 nanograms per milliliter (ng/mL) of Fibrinogen Equivalent Units (FEU). The reported units and the exact cutoff can vary based on the specific testing method used by the laboratory.
Modern medical practice uses age-adjusted D-dimer thresholds for older patients because levels tend to increase naturally with age. Using the fixed 500 ng/mL FEU cutoff in people over 50 can lead to unnecessary positive results. The age-adjusted cutoff is calculated by multiplying the patient’s age by 10 (Age x 10) for those above 50 years, often expressed in micrograms per liter (µg/L) FEU. For example, a 70-year-old patient would have a normal threshold of 700 µg/L FEU, improving the test’s accuracy in this population.
Clinical Significance of Elevated D-dimer Results
When a D-dimer result is above the normal threshold, it indicates increased clot formation and breakdown. The primary utility of the test is to help rule out serious thrombotic conditions, such as Venous Thromboembolism (VTE), Deep Vein Thrombosis (DVT), and Pulmonary Embolism (PE). A negative D-dimer result in a patient with low clinical suspicion provides a high negative predictive value, allowing providers to avoid invasive imaging tests.
Conversely, an elevated D-dimer level does not automatically confirm a clot, as the test is highly sensitive but lacks specificity. It suggests further investigation is necessary to locate the source of the increased clotting activity. Extremely high D-dimer levels may indicate a more severe condition, such as Disseminated Intravascular Coagulation (DIC).
The test is utilized as a triage tool, particularly in emergency settings, to quickly assess the risk of a blood clot. Interpretation must always be combined with a patient’s overall clinical picture and risk factors.
Non-Clotting Factors That Raise D-dimer
An elevated D-dimer result is not exclusively tied to the presence of a deep vein thrombosis or pulmonary embolism. Numerous physiological and pathological states unrelated to an acute clot can cause levels to rise, leading to a “false positive” result. Any condition activating the body’s inflammatory or coagulation systems can increase D-dimer levels.
Factors that commonly raise D-dimer levels include:
- Infections, such as sepsis or severe pneumonia, due to systemic inflammatory response.
- Recent trauma or surgery, which triggers tissue injury and subsequent fibrinolysis.
- Certain chronic conditions, including various forms of cancer and severe liver disease.
- Physiological states like pregnancy, often making the test unreliable for diagnosing VTE.