A D-dimer test is a blood test that measures a protein fragment created when a blood clot dissolves in the body. Healthcare professionals use this test to assess for blood clotting problems, frequently when a patient has symptoms of a serious condition like deep vein thrombosis (DVT) or a pulmonary embolism (PE).
The primary function of the D-dimer test is to help rule out the presence of such clots. A low or normal D-dimer level suggests a clotting problem is unlikely. A high level indicates that a clot might be present but does not confirm it, as other conditions can also raise D-dimer levels, making it a “rule-out” tool.
Standard D-dimer Reference Values
For healthy adults, particularly those under the age of 50, a standard reference value is used to interpret D-dimer test results. The most widely accepted cutoff is less than 0.50 micrograms per milliliter (mcg/mL) or less than 500 nanograms per milliliter (ng/mL). A result below this threshold is considered negative, indicating that a significant blood clot is highly improbable.
It is important to recognize that specific values and units can vary between laboratories. Results are commonly reported in either Fibrinogen Equivalent Units (FEU) or D-Dimer Units (D-DU). The 500 ng/mL cutoff is typically based on the FEU measurement, and this variability means that interpretation must always be done in the context of the specific laboratory’s established reference range.
When a patient’s D-dimer level falls below this value, clinicians can be more confident in ruling out conditions like DVT or PE without proceeding to more invasive imaging studies.
Age-Adjusted D-dimer Calculation
Research has shown that D-dimer levels naturally increase as people get older, independent of any acute clotting event. This physiological change means that if the standard cutoff of 500 ng/mL is applied universally, a larger number of older adults will have a “false positive” result. Such a result can trigger unnecessary anxiety and lead to further imaging tests, like CT scans, which carry their own risks.
To address this issue and improve the test’s accuracy in the elderly population, an age-adjusted calculation has been developed. For patients over the age of 50, the upper limit of the normal D-dimer range can be calculated by multiplying the patient’s age by 10 ng/mL. This formula is applicable when the laboratory uses the common ng/mL FEU (Fibrinogen Equivalent Units) scale.
The application of this formula is straightforward. For a 60-year-old individual, the adjusted cutoff would be 600 ng/mL (60 x 10). For a 75-year-old, the upper limit of normal would rise to 750 ng/mL (75 x 10). Using this tailored threshold helps clinicians more accurately rule out venous thromboembolism in older patients, reducing the number of follow-up imaging procedures.
Factors Other Than Age Affecting D-dimer Levels
While a blood clot is a primary cause for elevated D-dimer, several other medical situations can also lead to increased levels. This is because D-dimer is a marker of fibrin formation and degradation, a process not exclusive to dangerous clots. Understanding these other causes is important for interpreting a high D-dimer result. Common causes include:
- Pregnancy: The body prepares for childbirth by increasing its clotting potential, which results in a rise in D-dimer that increases progressively through each trimester.
- Recent major surgery or trauma: The body’s healing response involves forming clots to repair damaged blood vessels. As these clots dissolve, D-dimer is released into the bloodstream.
- Active cancer: Cancer can trigger a pro-thrombotic state, leading to chronically elevated levels.
- Widespread infection or sepsis: These conditions can activate the body’s clotting system and result in a similar effect on D-dimer measurements.
- Chronic inflammatory diseases: Conditions such as rheumatoid arthritis or lupus can also cause higher D-dimer values.
Clinical Interpretation and Next Steps
The D-dimer test is characterized by high sensitivity, meaning a negative result reliably rules out a clot. However, its low specificity means a positive result is not a definitive diagnosis, as many other factors can cause an elevation. The clinical pathway following a D-dimer test depends on the result in the context of the patient’s risk profile.
If the D-dimer level is below the established cutoff—either the standard or the age-adjusted value—the likelihood of a dangerous clot like a DVT or PE is considered very low. In this scenario, the investigation for a clot is concluded.
If the D-dimer level is elevated above the appropriate threshold, it signals the need for further diagnostic action. An elevated result indicates that a clot cannot be ruled out, and the next step is an imaging study to visualize the blood vessels directly.
For a suspected DVT, the most common follow-up test is a venous ultrasound of the limbs. For a suspected PE, the standard procedure is a CT pulmonary angiography (CTPA) scan, which provides a detailed view of the pulmonary arteries.