D-dimer

D-dimer is a cross-linked fibrin degradation product generated when plasmin cleaves stabilised fibrin, and serves as a global marker of activated coagulation and fibrinolysis. It is measured by latex-enhanced immunoturbidimetric or ELISA-based assays, reported either in fibrinogen-equivalent units (FEU) or D-dimer units (DDU). Its principal clinical role is ruling out venous thromboembolism: a value below the assay-specific cutoff (commonly <500 ng/mL FEU) combined with a low pre-test probability has a negative predictive value above 95%. The ADJUST-PE study (Righini 2014) validated an age-adjusted cutoff (age x 10 ng/mL FEU for patients >50 years), which safely increases specificity in older adults. D-dimer rises physiologically with age, pregnancy, inflammation, surgery, trauma, malignancy, sepsis, and severe liver or kidney disease, so a positive result is non-specific and must be interpreted alongside imaging.