A D-dimer test measures a protein fragment produced when a blood clot dissolves in the body. It is used in emergency settings to help determine if a blood clot is present, such as a deep vein thrombosis (DVT) or a pulmonary embolism (PE). While this test is a useful screening tool, its interpretation is not always straightforward in certain patient populations. An approach known as age-adjustment was developed to refine the test’s application by accounting for naturally occurring changes in the body to improve the diagnostic process.
The Challenge with Standard D-Dimer Testing in Older Adults
The primary issue with conventional D-dimer testing arises in older adults. As individuals age, their baseline D-dimer levels tend to increase even without a harmful blood clot. This is attributed to factors like a higher prevalence of coexisting medical conditions and low-grade inflammation. Consequently, the standard fixed cutoff value for a “positive” test, commonly set at 500 nanograms per milliliter (ng/mL), becomes less reliable.
Using this single threshold for all adults results in a higher rate of false-positive results among the elderly. The test’s ability to correctly identify someone without a clot, known as specificity, can drop in patients over 80 years old. This low specificity means many older patients have an elevated D-dimer level that is not caused by a DVT or PE.
These false positives have downstream consequences. An elevated D-dimer prompts further investigation, most often with advanced imaging like a computed tomography pulmonary angiogram (CTPA). This exposes patients to ionizing radiation and potential kidney damage from contrast dye, while also inducing patient anxiety and increasing healthcare costs.
Calculating the Age-Adjusted Cutoff
To address the high rate of false positives in older individuals, a formula was developed to create a personalized D-dimer cutoff for patients over the age of 50. The most widely used formula for calculating the age-adjusted D-dimer threshold is the patient’s age multiplied by 10 ng/mL. This calculation generates a new, higher cutoff that is more appropriate for the individual’s age.
For example, under the standard protocol, a 75-year-old patient with a D-dimer level of 650 ng/mL would have a positive result, as it is above the 500 ng/mL threshold. Using the age-adjusted formula, the new cutoff for this patient would be 750 ng/mL (75 years × 10). In this scenario, their result of 650 ng/mL would be considered negative, potentially avoiding subsequent imaging.
Different laboratories may use different measurement units, which can alter the formula. While the “Age x 10” rule applies to labs using fibrinogen equivalent units (FEUs), another common unit is the D-dimer unit (DDU). For assays reporting in DDUs, the calculation is Age x 5. A healthcare professional must use the correct formula based on the specific assay being used at their institution.
Clinical Utility and Improved Diagnostic Accuracy
The main benefit of using an age-adjusted D-dimer cutoff is improved diagnostic performance for older adults. This approach increases the test’s specificity—its ability to correctly identify patients who do not have a venous thromboembolism (VTE). Studies show that for patients over 80, specificity can improve without a meaningful loss in sensitivity, which is the test’s ability to correctly identify patients who do have a clot.
This enhanced specificity allows clinicians to more confidently rule out conditions like PE or DVT in a larger number of older patients without ordering more tests. By raising the threshold for a positive result, the age-adjusted method reduces the number of false positives that trigger unnecessary follow-up procedures, leading to a reduction in the use of CTPA scans.
Reducing the number of imaging studies provides multiple advantages. It lowers overall healthcare expenditures and minimizes patient exposure to ionizing radiation. It also reduces the risk of contrast-induced kidney problems, which is a particular concern for older individuals who may have pre-existing renal issues.
Appropriate Use and Limitations
The age-adjusted D-dimer must be applied in the correct clinical context. Its greatest value is as a “rule-out” test for patients considered to have a low or intermediate pre-test probability of having a VTE. This initial risk assessment is determined by a healthcare provider using established clinical scoring systems, such as the Wells’ Criteria or the Pulmonary Embolism Rule-out Criteria (PERC).
The test is not used for patients who have a high clinical suspicion of a blood clot. In high-risk scenarios, the likelihood of a clot is strong enough that imaging is warranted regardless of the D-dimer result. Applying the age-adjusted D-dimer in this population would not change clinical management and would only delay necessary diagnostic imaging.
A VTE is not the only condition that can cause D-dimer levels to rise. Other situations can also lead to an elevated result, including:
- Recent major surgery
- Trauma
- Active cancer
- Infection
- Pregnancy
For this reason, a healthcare professional must consider the age-adjusted D-dimer value within the broader context of the patient’s overall health, medical history, and clinical presentation.