In a hemodynamically stable patient with suspected pulmonary embolism and low pretest probability, first-step test?

• A. V/Q scan
• B. D-dimer testing
• C. CT pulmonary angiography
• D. Pulmonary function testing

Answer: (B)

In a patient who is hemodynamically stable with suspected pulmonary embolism and has a low pretest probability, the goal is to safely rule out PE without exposing them to unnecessary imaging. D-dimer testing fits this role because it is highly sensitive for acute PE. A negative D-dimer effectively rules out PE in low-risk patients, allowing clinicians to avoid further imaging and its radiation and contrast exposure. If the D-dimer is positive, imaging—such as CT pulmonary angiography or a ventilation-perfusion scan—is then pursued to confirm the diagnosis. D-dimer, however, has limited specificity, so a positive result does not confirm PE on its own and prompts further imaging. Other tests like CT pulmonary angiography are more informative but are not appropriate as the initial step in a low-probability scenario, since they expose the patient to radiation and contrast unnecessarily if PE can be ruled out with a negative D-dimer. Pulmonary function testing isn’t useful for acute PE evaluation.