DDimerTests

D-Dimer tests, often written as D-dimer tests or D-dimer assays, are laboratory tests that measure the level of a specific fibrin degradation product in the blood. D-dimer is produced when cross-linked fibrin is broken down by the body's fibrinolytic system, so its presence indicates recent or ongoing formation and breakdown of clots. The test is used as a marker of activation of coagulation and fibrinolysis rather than as a stand-alone diagnostic there for specific conditions.

The primary clinical use of the D-dimer test is in the evaluation of suspected venous thromboembolism, including deep vein thrombosis and pulmonary embolism, and in some settings disseminated intravascular coagulation. It is commonly employed to help rule out these conditions in patients with low to intermediate pretest probability. A negative result can substantially reduce the likelihood of a thrombotic event, potentially avoiding imaging studies, whereas a positive result does not confirm a thrombus and must be interpreted in the context of the patient's clinical risk and imaging findings.

Interpretation of D-dimer results depends on the assay and the reported units. Immunoassays are highly sensitive but less specific, leading to false positives in conditions such as pregnancy, infection, inflammation, recent surgery, cancer, liver disease, and advanced age. Thresholds vary; a common cutoff is 500 ng/mL fibrinogen-equivalent units (FEU), but many laboratories report different units (such as D-dimer units) and may use age-adjusted thresholds to improve specificity in older patients.

D-dimer testing should be integrated with clinical probability assessment and, when indicated, followed by appropriate imaging to confirm or exclude thromboembolism. It is not diagnostic on its own and can be influenced by a wide range of non-thrombotic conditions and laboratory factors.