What is the definitive test for diagnosing pulmonary embolism (PE) in a patient with a potential history of COVID-19?

Definitive Test for Pulmonary Embolism

Computed tomographic pulmonary angiography (CTPA) is the definitive test for diagnosing pulmonary embolism, with a sensitivity of 83% and specificity of 96%, and is the method of choice for imaging the pulmonary vasculature. 1

Diagnostic Algorithm

Step 1: Clinical Probability Assessment

• Always begin with pre-test probability assessment using validated scoring systems (Wells score or revised Geneva score) before ordering imaging. 2
• This stratifies patients into low, intermediate, or high clinical probability categories. 1, 2

Step 2: D-dimer Testing (For Low/Intermediate Probability Only)

• If clinical probability is low or intermediate, measure D-dimer first. 2
• A negative D-dimer excludes PE in these patients—no further testing needed. 2
• Do not use D-dimer in high clinical probability patients or hospitalized patients where its utility is limited. 2
• Age-adjusted D-dimer cutoffs (age × 10 mg/L for patients >50 years) improve specificity in older patients. 1

Step 3: Imaging with CTPA

• CTPA is the gold standard imaging test when D-dimer is positive or clinical probability is high. 1, 2
• CTPA visualizes pulmonary arteries down to the subsegmental level with excellent accuracy. 1
• The test has strong validation in prospective management outcome studies and low inconclusive rates (3-5%). 1

Alternative Imaging Modalities

When to Use V/Q Scanning Instead

• Consider V/Q scintigraphy in specific populations: 1
  • Young patients (especially females) to minimize radiation exposure to breast tissue
  • Pregnant women
  • Patients with contrast allergy or severe renal failure
  • Patients with normal chest X-ray
• V/Q scanning has lower radiation (2 mSv vs 3-10 mSv for CTPA) but higher inconclusive rate (50%). 1

Adjunctive Testing

• Doppler ultrasound of lower extremities can support the diagnosis when CTPA is contraindicated, as finding proximal DVT confirms need for anticoagulation. 1
• Point-of-care bedside ultrasound or echocardiography may be combined with standard testing in unstable patients. 1

Special Considerations for COVID-19 Patients

• Use standard-of-care objective testing (CTPA, V/Q scan, MRI venography, Doppler ultrasound) based on clinical suspicion. 1
• Do not perform routine screening with bedside Doppler or based solely on elevated D-dimer levels. 1
• COVID-19 patients have markedly elevated D-dimer levels (87.3% in one study) that may not correlate with PE presence. 3
• PE incidence in hospitalized COVID-19 patients is approximately 25%, significantly higher than typical medical patients. 4
• A D-dimer threshold >1600 ng/mL showed 100% sensitivity for PE in COVID-19 patients in external validation. 4

Critical Pitfalls to Avoid

• Never rely on CTPA alone when there is discordance with clinical probability. If clinical probability is high but CTPA is negative, consider further testing as negative predictive value drops to 60%. 1
• The clinical relevance of isolated subsegmental PE on CTPA remains uncertain—correlate with clinical context. 1
• Avoid ordering CTPA in low-probability patients with negative D-dimer—this leads to overdiagnosis and unnecessary anticoagulation risks. 2
• In hemodynamically unstable patients (shock/hypotension), if CTPA is not immediately available or patient is too unstable for transport, perform bedside echocardiography to assess for right ventricular strain. 2
• Pulmonary angiography, while historically the gold standard, is invasive, not readily available, and carries highest radiation exposure (10-20 mSv). 1