What is the best initial imaging test to rule out pulmonary embolism (PE) in a patient with exertional dyspnea and chest pain?

Diagnostic Testing for Pulmonary Embolism in This Patient

You should calculate a clinical probability score (Wells or Geneva) first, then order plasma D-dimer testing if the patient has low-to-intermediate probability, or proceed directly to CT pulmonary angiography if high probability is established. 1

Why Risk Stratification Must Come First

This patient requires formal pretest probability assessment before selecting any diagnostic test. 1 The American College of Physicians explicitly recommends using validated clinical prediction rules (Wells score or revised Geneva score) to stratify pretest probability before proceeding with any testing. 1

The PERC rule cannot be applied to this patient because he fails multiple criteria: age >50 years (he is 64), and potentially heart rate ≥100 bpm (pulse is 95, borderline). 1 All eight PERC criteria must be met to safely exclude PE without further testing. 1

Calculating Pretest Probability

Apply the revised Geneva score, which assigns points for:

• Age >65 years: This patient is 64, so no points
• Heart rate: 95 bpm (no points if <95,3 points if 75-94,5 points if ≥95)
• Previous PE/DVT: None documented (0 points)
• Recent surgery: CABG was 5 years ago (0 points)
• Hemoptysis: None (0 points)
• Active cancer: None mentioned (0 points)
• Unilateral leg pain/swelling: None (0 points) 1

This patient likely falls into low-to-intermediate probability based on minimal risk factors. 1

The Correct Diagnostic Pathway

For Low-to-Intermediate Probability (Most Likely This Patient):

Order plasma D-dimer testing first (Answer A). 2 The American College of Physicians specifically recommends obtaining high-sensitivity D-dimer as the initial diagnostic test in patients with intermediate pretest probability, and imaging should not be used as the initial test in these patients. 2

• If D-dimer is negative using age-adjusted cutoff (64 × 10 = 640 ng/mL), PE is safely excluded without imaging. 1, 2
• If D-dimer is positive, proceed to CT pulmonary angiography. 1, 2

For High Probability (If Score Indicates >40% Probability):

Proceed directly to CT pulmonary angiography (Answer C) without D-dimer testing. 1 A negative D-dimer will not obviate the need for imaging in high-probability patients, making it an unnecessary step. 1

Why Each Answer Is Right or Wrong

Answer A (Plasma D-dimer) is correct if pretest probability is low-to-intermediate, which is most likely given this patient's presentation. 2 This prevents unnecessary radiation exposure and contrast nephropathy risk in patients who can be safely excluded with negative D-dimer. 2

Answer C (CT pulmonary angiography) is correct only if pretest probability is high (>40%). 1 Starting with CTPA in moderate-probability patients exposes them to unnecessary radiation and contrast risks when D-dimer could safely exclude PE. 2

Answer B (V/Q scan) is reserved for patients with contraindications to CT (renal insufficiency, contrast allergy, pregnancy) or when CT is unavailable. 1 V/Q scanning is diagnostic in only 30-50% of cases, often yielding non-diagnostic results requiring further testing. 1

Answer D (No further testing) is incorrect because this patient does not meet all eight PERC criteria and therefore requires diagnostic evaluation. 1

Critical Pitfalls to Avoid

• Do not skip risk stratification. The choice between D-dimer and direct imaging depends entirely on pretest probability. 1
• Do not wait for troponin results before ordering PE workup, as acute coronary syndrome and PE can coexist. 1
• Do not use the generic 500 ng/mL D-dimer cutoff in this 64-year-old patient; use age-adjusted threshold of 640 ng/mL to increase specificity without missing cases. 1, 2
• Do not use D-dimer as a screening test in high-probability patients, as it has low negative predictive value in this population. 1

The Most Practical Approach

In real-world emergency department practice, if you must choose one test immediately without formal scoring: Order plasma D-dimer (Answer A) for this patient with no clear high-risk features, as this represents the safest initial approach that avoids unnecessary imaging in the majority of patients while maintaining high sensitivity for PE. 2, 3