ddimer

D-dimer is a small protein fragment that results from the breakdown of cross-linked fibrin during blood clot dissolution. It is released into the bloodstream when a clot forms and is subsequently degraded by plasmin. The presence of elevated D-dimer indicates recent or ongoing coagulation and fibrinolysis, but it does not specify the location or cause of a clot.

Clinically, D-dimer testing is primarily used to help assess the probability of venous thromboembolism, including deep vein thrombosis and pulmonary embolism, in conjunction with pretest clinical assessment tools such as the Wells score. A negative D-dimer test can help exclude VTE in patients with a low pretest probability, potentially reducing the need for imaging. A positive result is not diagnostic by itself and typically prompts further diagnostic imaging or clinical evaluation.

D-dimer is measured with quantitative immunoassays in blood samples. Results are reported in units such as fibrinogen-equivalent units (FEU) or D-dimer units (DDU), often expressed in ng/mL. A common reference threshold is 500 ng/mL FEU, though many laboratories report local cutoffs. In older patients or those with comorbidities, baseline D-dimer levels may be elevated, and age-adjusted thresholds (for example, age × 10 ng/mL in FEU for patients over 50) are sometimes used to improve specificity.

Limitations include its lack of specificity: many conditions, including pregnancy, infection, inflammation, cancer, recent surgery, and liver disease, can raise D-dimer levels. Conversely, a negative test does not completely rule out thrombosis in patients with high clinical suspicion.