What labs and tests are needed to diagnose pulmonary embolism?

Diagnostic Testing for Pulmonary Embolism

Initial Risk Stratification is Mandatory

The first step in diagnosing pulmonary embolism is to assess clinical probability using validated prediction rules (such as Wells score or Geneva score) to stratify patients into low, intermediate, or high pretest probability categories. 1 This stratification determines the entire diagnostic pathway and prevents unnecessary testing while ensuring appropriate workup.

Diagnostic Algorithm Based on Clinical Probability

For Low or Intermediate Clinical Probability

Start with high-sensitivity D-dimer measurement as the initial diagnostic test—imaging should NOT be the first test in these patients. 1

• If D-dimer is negative (below age-adjusted cutoff), pulmonary embolism is excluded and no further testing is needed. 1
• Use age-adjusted D-dimer thresholds (age × 10 ng/mL) in patients older than 50 years rather than the generic 500 ng/mL cutoff, as this improves specificity without sacrificing safety 1
• If D-dimer is elevated, proceed directly to CT pulmonary angiography (CTPA) 1

Special exception: In patients meeting ALL Pulmonary Embolism Rule-Out Criteria (PERC), do not obtain D-dimer or imaging studies at all 1

For High Clinical Probability

Proceed directly to CT pulmonary angiography without D-dimer testing, as a normal D-dimer does not safely exclude PE even with highly sensitive assays in this population. 1

For Hemodynamically Unstable Patients (Shock or Hypotension)

Perform bedside transthoracic echocardiography as the most useful initial test. 1, 2

• Evidence of right ventricular dysfunction on echocardiography in an unstable patient is sufficient to prompt immediate reperfusion treatment without further testing 1
• If the patient is stable enough and echocardiography shows RV dysfunction, proceed to emergency CTPA for definitive diagnosis 1

Primary Imaging Modality

CT pulmonary angiography (CTPA) is the imaging test of choice for diagnosing pulmonary embolism. 1, 2

• CTPA visualizes pulmonary arteries down to the subsegmental level with sensitivity of 83% and specificity of 96% 1
• A normal CTPA safely excludes PE in patients with low or intermediate clinical probability 1
• A segmental or more proximal filling defect on CTPA confirms PE in patients with intermediate or high clinical probability 1

Important caveat: In patients with high clinical probability and negative CTPA, consider further testing as the negative predictive value drops to only 60% in this group 1

Alternative Imaging: Ventilation-Perfusion (V/Q) Scanning

Reserve V/Q scintigraphy for patients with contraindications to CTPA (iodine allergy, severe renal failure, pregnancy) or when CTPA is unavailable. 1

• A normal perfusion scan excludes PE 1
• A high-probability V/Q scan confirms PE 1
• V/Q scanning has lower radiation exposure (~2 mSv) compared to CTPA (3-10 mSv), making it preferable in young patients and pregnancy 1
• Major limitation: Non-diagnostic results occur in approximately 50% of cases 1

Compression Ultrasonography of Lower Extremities

Lower limb compression ultrasonography (CUS) showing proximal deep vein thrombosis confirms venous thromboembolism and justifies anticoagulation without further PE testing. 1

• CUS detects DVT in 30-50% of patients with PE 1
• This test may be considered in selected patients to obviate the need for CTPA if positive 1
• If only distal DVT is found, further testing should be performed to confirm PE 1
• A negative CUS does not exclude PE, as 50% of PE patients have normal lower extremity ultrasound 1

Tests NOT Recommended for Diagnosis

• Echocardiography should NOT be used systematically for diagnosis in hemodynamically stable patients (sensitivity only 60-70%) 1
• Magnetic resonance angiography is NOT recommended for ruling out PE 1
• CT venography as an adjunct to CTPA is NOT recommended 1
• Routine blood gas analysis lacks sufficient sensitivity and specificity to confirm or exclude PE 3

Critical Pitfalls to Avoid

• Never skip clinical probability assessment—proceeding directly to imaging without risk stratification leads to overuse of CT and unnecessary radiation exposure 1
• Do not order D-dimer in high-probability patients—it will not change management and wastes time 1
• Beware of subsegmental filling defects on CTPA—their clinical significance is uncertain and further testing may be needed 1
• Do not rely on chest X-ray, ECG, or troponin for diagnosis—these are neither sensitive nor specific for PE, though troponin and BNP are useful for prognostic stratification after diagnosis is confirmed 1, 3