What is the recommended initial diagnostic test for a patient suspected of having a pulmonary embolism?

Diagnostic Testing for Pulmonary Embolism

CT pulmonary angiography (CTPA) is the imaging test of choice for diagnosing pulmonary embolism in most patients, but the initial diagnostic approach depends on clinical probability assessment—not all patients require imaging. 1

Risk Stratification First

Before ordering any test, you must stratify pretest probability using validated clinical prediction rules:

• Use the Wells score or revised Geneva score to categorize patients into low, intermediate, or high probability groups 1, 2
• Clinical gestalt by experienced clinicians performs equally well as formal scoring systems, with 97% sensitivity 2
• This stratification determines the entire diagnostic pathway and prevents unnecessary testing 1

Low Pretest Probability Patients

Apply the Pulmonary Embolism Rule-Out Criteria (PERC) first in patients with low clinical probability:

• If all 8 PERC criteria are met (age <50, pulse <100, O₂ sat >94%, no hemoptysis, no estrogen use, no prior VTE, no unilateral leg swelling, no recent surgery/trauma), no further testing is needed—neither D-dimer nor imaging 1, 2
• PERC has 97% sensitivity and safely excludes PE without any testing 2

If PERC criteria are NOT met, obtain high-sensitivity D-dimer:

• A negative D-dimer (using age-adjusted thresholds: age × 10 ng/mL for patients >50 years) excludes PE without imaging 1, 2
• Age-adjusted thresholds increase the proportion of patients in whom PE can be excluded from 6.4% to 30% 2
• Do not proceed to imaging if D-dimer is below the age-adjusted cutoff 1

Intermediate Pretest Probability Patients

Obtain high-sensitivity D-dimer as the initial test:

• If negative (using age-adjusted thresholds), PE is excluded—no imaging needed 1, 2
• If positive, proceed to CTPA 1
• Do not use imaging as the initial test in intermediate probability patients 1

High Pretest Probability Patients

Proceed directly to CTPA without D-dimer testing:

• A negative D-dimer cannot safely exclude PE in high-probability patients due to low negative predictive value 1, 2
• CTPA has 83% sensitivity and 96% specificity for PE 2
• Reserve ventilation-perfusion (V/Q) scans for patients with contraindications to CTPA (renal failure, contrast allergy, pregnancy) or when CTPA is unavailable 1

Hemodynamically Unstable Patients (Shock/Hypotension)

This is a distinct clinical scenario requiring immediate action:

• Perform bedside echocardiography if CTPA is not immediately available or the patient is too unstable for transport 1, 2
• Echocardiography showing RV overload/dysfunction in the appropriate clinical context justifies PE-specific treatment 1
• If the patient can be stabilized, obtain CTPA for definitive diagnosis 1
• Initiate anticoagulation immediately without waiting for imaging confirmation in hemodynamically unstable patients 2

Why CTPA is the Preferred Imaging Test

CTPA has become the standard imaging modality because:

• Sensitivity >95% for segmental or larger emboli 2, 3, 4
• Provides alternative diagnoses when PE is absent (pneumonia, aortic dissection, cardiac pathology) 1, 2
• Good interobserver agreement even with relatively inexperienced readers 1
• Most cost-effective strategy in diagnostic algorithms 2
• Negative CTPA safely excludes PE with 3-month thromboembolic risk <1% 1, 4

Alternative Imaging: V/Q Scanning

V/Q scanning remains valid in specific circumstances:

• Preferred in younger patients and women to avoid radiation exposure and breast cancer risk 1, 5
• Diagnostic (normal or high-probability) in 30-50% of cases 1
• Higher diagnostic yield in patients with normal chest X-ray 1
• Reserve for contraindications to CTPA: renal failure, contrast allergy, pregnancy 1

Critical Pitfalls to Avoid

Do not obtain D-dimer in high-probability patients—it will not change management and delays definitive diagnosis 1, 2

Do not use D-dimer as a screening test in hospitalized patients—it is frequently elevated from comorbid conditions, surgery, infection, and inflammation, with fewer than 10% having negative results 2

Do not skip clinical probability assessment—proceeding directly to imaging without risk stratification leads to overuse of CT and unnecessary radiation exposure 1

In patients with negative CTPA but high clinical probability, consider additional testing (lower extremity ultrasound, V/Q scan) before definitively ruling out PE, as CTPA has a false-negative rate of 5-17% 2

Lower extremity compression ultrasound can be useful as an initial test in patients with obvious DVT signs, as finding proximal DVT is sufficient to warrant anticoagulation without further testing 1, 2