What is the recommended initial scan for a patient with suspected pulmonary embolism (PE)?

CT Pulmonary Angiography (CTPA) is the Recommended Initial Scan for Suspected Pulmonary Embolism

CTPA is now the recommended initial lung imaging modality for non-massive pulmonary embolism (PE). 1

Rationale for CTPA as First-Line Imaging

CTPA has become the gold standard for diagnosing PE due to several advantages:

• High diagnostic accuracy with sensitivity of 83% and specificity of 96% 2
• Allows visualization of pulmonary arteries down to the subsegmental level 2
• Provides direct visualization of intravascular thrombus 1
• Can identify alternative diagnoses in approximately 35% of patients without PE 2
• Good interobserver agreement even with relatively inexperienced assessors 1
• Safe to withhold anticoagulation when PE is excluded on CTPA (only 1.1% subsequent PE by 3 months) 1

Diagnostic Algorithm for Suspected PE

1. Assess clinical probability using validated tools (Wells score or Geneva score) 2

   • Document clinical probability for all patients

2. D-dimer testing for patients with low or intermediate clinical probability 2

   • A negative D-dimer reliably excludes PE in these patients without need for imaging 1, 2
   • Do not perform D-dimer in high clinical probability patients 2

3. Initial imaging based on clinical scenario:

   • Standard case: CTPA is the recommended first-line imaging 1, 2, 3
   • If signs/symptoms of DVT: Consider lower extremity ultrasound first 2
   • If massive PE suspected: CTPA or echocardiography can reliably diagnose 1

Alternative Imaging Options

Consider alternative imaging in specific situations:

• Ventilation-perfusion (V/Q) scan when:

  • CTPA is contraindicated (severe renal failure, contrast allergy)
  • During pregnancy (to reduce fetal radiation exposure) 2
  • When CTPA is unavailable 4

• Lower extremity ultrasound when:

  • Clinical signs/symptoms of DVT are present 2
  • Finding a proximal DVT is sufficient to warrant anticoagulation 2

Important Caveats

• A good quality negative CTPA reliably excludes PE and no further investigation is needed 1
• CTPA may miss isolated subsegmental PE, though the clinical significance is uncertain 2
• Technical factors affect CTPA quality: thin section collimation, proper timing of contrast, and viewing at workstations improve results 1
• The negative predictive value of CTPA varies with clinical probability (96% in low probability but only 60% in high probability patients) 2
• A single normal leg ultrasound should not be relied upon to exclude subclinical DVT 2

Conclusion

The British Thoracic Society guidelines and more recent evidence strongly support CTPA as the initial imaging modality of choice for suspected PE, with alternatives reserved for specific clinical scenarios. The diagnostic approach should be guided by clinical probability assessment and appropriate use of D-dimer testing before proceeding to imaging.