If CT angiography is contraindicated, which diagnostic test is used to evaluate suspected pulmonary embolism?

• A. Ventilation-perfusion (V/Q) scan
• B. Pulmonary angiography
• C. Echocardiography
• D. D-dimer testing

Answer: (A)

When CT angiography can’t be used, the best alternative to evaluate suspected pulmonary embolism is a ventilation-perfusion scan. This nuclear medicine test separately assesses airflow (ventilation) and blood flow (perfusion) in the lungs. In a PE, a part of the lung loses its blood supply while ventilation remains, creating a mismatch between ventilation and perfusion that the scan can detect.

A high-probability V/Q result strongly supports PE, guiding diagnosis without the need for contrast. If the V/Q result is low-probability or nondiagnostic, interpretation becomes more challenging, and other approaches may be considered, but the scan remains the key noninvasive option when CT with contrast isn’t available or contraindicated.

In contrast, pulmonary angiography is more invasive and reserved for cases where noninvasive testing is inconclusive. Echocardiography can show signs of right heart strain from PE but does not diagnose PE itself. D-dimer testing helps rule out PE in low–pretest-probability scenarios but is not diagnostic on its own in the setting of high suspicion.