What is the best test for suspected pulmonary embolus (embolus)?

Diagnostic Approach for Suspected Pulmonary Embolism

CT pulmonary angiography (CTPA) is the best test for suspected pulmonary embolism in patients with high pretest probability, while D-dimer testing should be used first in patients with low or intermediate pretest probability. 1

Clinical Probability Assessment First

Before ordering any diagnostic test, clinicians should determine the pretest probability of PE using validated clinical decision tools:

• Wells score or revised Geneva score to stratify patients into low, intermediate, or high probability categories 2
• Pulmonary Embolism Rule-Out Criteria (PERC) for patients with low clinical probability:
  • Age < 50 years
  • Heart rate < 100 beats/minute
  • Oxygen saturation ≥ 95%
  • No history of venous thromboembolism
  • No recent trauma or surgery
  • No hemoptysis
  • No estrogen use
  • No unilateral lower limb edema

Diagnostic Algorithm Based on Clinical Probability

Low Pretest Probability

• If all 8 PERC criteria are met: No further testing needed 1, 2
• If PERC criteria not all met: Order high-sensitivity D-dimer test 1
  • If D-dimer negative: No imaging needed 1
  • If D-dimer positive: Proceed to CTPA 1

Intermediate Pretest Probability

• Order high-sensitivity D-dimer test 1, 2
  • If D-dimer negative: No imaging needed 1
  • If D-dimer positive: Proceed to CTPA 1

High Pretest Probability

• Proceed directly to CTPA without D-dimer testing 1, 2
• If CTPA is contraindicated or unavailable: Use ventilation-perfusion (V/Q) scan 1

Important Considerations for D-dimer Testing

• Use age-adjusted D-dimer thresholds (age × 10 ng/mL rather than generic 500 ng/mL) for patients older than 50 years 1, 2
• D-dimer testing has limited utility in hospitalized patients due to high false-positive rates 1
• Never order D-dimer in high pretest probability patients as negative results won't change management 1

Imaging Options

CT Pulmonary Angiography (CTPA)

• First-line imaging test when indicated 1, 2
• High sensitivity (95%) and specificity (98%) 2
• Considered the gold standard diagnostic technique in emergency departments 3
• Can visualize alternative diagnoses if PE is not present

Ventilation-Perfusion (V/Q) Scan

• Alternative when CTPA is contraindicated (contrast allergy, renal failure) or unavailable 1
• Preferred in younger patients, especially females, to reduce radiation exposure to breast tissue 1
• Diagnostic in 30-50% of emergency department patients with suspected PE 1
• More useful when chest X-ray is normal 1

Compression Ultrasound (CUS)

• Can be useful in specific circumstances:
  • Finding proximal DVT in a patient with suspected PE is sufficient to warrant anticoagulation without further testing 1
  • May be an option before CTPA in patients with contraindications to CT 1
• Limited to four-point examination (groin and popliteal fossa) 1

Common Pitfalls to Avoid

• Overuse of CTPA: Studies show PE is confirmed in only a minority of patients undergoing CTPA 4, 5
• Skipping clinical probability assessment: Leads to unnecessary testing and potential misdiagnosis 2
• Ordering D-dimer in high probability patients: Wastes resources and may lead to false negatives 1, 2
• Failure to use age-adjusted D-dimer thresholds: Decreases specificity in older patients 1, 2
• Applying PERC criteria to non-low probability patients: PERC should only be used in low clinical probability cases 2

Special Situations

Pregnancy

• V/Q scan is preferred over CTPA in early pregnancy due to radiation exposure concerns 2

Shock or Hypotension (High-Risk PE)

• Emergency CTPA or bedside echocardiography based on availability 1
• Echocardiographic evidence of right ventricular dysfunction may prompt immediate treatment in unstable patients 1

By following this evidence-based approach, clinicians can accurately diagnose PE while minimizing unnecessary imaging and its associated risks and costs.