What is the initial test for suspected pulmonary embolism (PE)?

Initial Testing for Suspected Pulmonary Embolism

Computed Tomography Pulmonary Angiography (CTPA) is the recommended initial lung imaging modality for non-massive pulmonary embolism (PE), following appropriate clinical probability assessment and D-dimer testing when indicated. 1, 2

Diagnostic Algorithm

Step 1: Clinical Probability Assessment

• All patients with possible PE should have clinical probability assessed and documented using validated clinical prediction rules (Wells score or Geneva score) 2, 1
• Categorize patients into low, intermediate, or high pretest probability of PE 1

Step 2: D-dimer Testing

• For low pretest probability patients:
  • If all Pulmonary Embolism Rule-Out Criteria (PERC) are met: No further testing needed 1
  • If PERC criteria not all met: Proceed to D-dimer testing 1
• For intermediate pretest probability patients:
  • Obtain high-sensitivity D-dimer measurement 1, 2
• For high pretest probability patients:
  • Skip D-dimer testing and proceed directly to imaging 1, 2

Step 3: D-dimer Interpretation

• For patients >50 years: Use age-adjusted D-dimer thresholds (age × 10 ng/mL) 1
• If D-dimer is below threshold: No further testing needed 1, 2
• If D-dimer is above threshold: Proceed to imaging 1, 2

Step 4: Imaging

• CTPA is the recommended initial imaging test for patients requiring imaging based on clinical probability and D-dimer results 1, 2
• Ventilation-perfusion (V/Q) scanning should be reserved for patients with contraindications to CTPA (severe renal failure, contrast allergy, pregnancy) 1, 2

Benefits of CTPA

• High sensitivity (83%) and specificity (96%) 1
• Allows visualization of pulmonary arteries down to the segmental level 1
• Can identify alternative diagnoses in approximately 35% of patients without PE 1
• Good interobserver agreement even with relatively inexperienced assessors 2

Special Considerations

For patients with clinical DVT

• Leg ultrasound as the initial imaging test may be sufficient to confirm venous thromboembolism (VTE) 2
• However, a single normal leg ultrasound should not be relied upon for exclusion of subclinical DVT 2

For massive PE

• CTPA or echocardiography will reliably diagnose clinically massive PE 2
• Imaging should be performed within 1 hour in massive PE 2

For pregnant women

• Consider V/Q scan to reduce fetal radiation exposure 1

Common Pitfalls and Caveats

• D-dimer testing should not be performed in those with high clinical probability of PE 2
• A negative D-dimer test reliably excludes PE only in patients with low or intermediate clinical probability 2
• CTPA may miss isolated subsegmental PE, with uncertain clinical significance 1
• The negative predictive value of CTPA varies with clinical probability (96% in low probability patients but only 60% in high probability patients) 1
• Hospitals should provide information on sensitivity and specificity of their D-dimer test 2

By following this evidence-based approach to diagnosing PE, clinicians can optimize patient outcomes while minimizing unnecessary testing and radiation exposure.