In a patient with suspected deep venous thrombosis, which initial test has high sensitivity to rule out DVT?

• A. D-dimer
• B. Venous duplex ultrasound
• C. CT venography
• D. MRI

Answer: (A)

D-dimer is used as a rule-out test because it has high sensitivity for deep venous thrombosis. It measures a fibrin degradation product released when clots are formed and broken down. A negative D-dimer makes DVT unlikely in patients with low to intermediate pretest probability, so it can safely exclude DVT without imaging. A positive result isn’t specific for DVT—many conditions can raise D-dimer—so it must be followed by imaging, typically compression duplex ultrasound, to confirm whether a clot is present. Imaging studies like duplex ultrasound are more definitive but are used after D-dimer when the pretest probability isn’t clearly low. In short, the D-dimer’s high sensitivity makes it the best initial test to help rule out DVT.