Diagnostic Approach for Suspected Pulmonary Embolism
You should order a plasma D-dimer test as the initial diagnostic test to help rule out pulmonary embolism in this patient. 1
Risk Stratification First
Before ordering any test, you must establish the pretest probability of PE using a validated clinical prediction rule (Wells or Geneva score) or clinical gestalt 1. This patient presents with:
- Exertional dyspnea and chest pain
- History of CAD with prior CABG (alternative diagnosis possible)
- No history of thromboembolic disease
- Clear lungs on examination
- Normal sinus rhythm without ST changes on EKG
This clinical picture suggests a low-to-intermediate pretest probability of PE, as the symptoms could be cardiac in origin given his CAD history, but PE cannot be excluded without objective testing 1.
Why D-Dimer is the Correct Initial Test
For patients with low or intermediate pretest probability of PE, high-sensitivity D-dimer measurement should be the initial diagnostic test—not imaging studies 1. The American College of Physicians explicitly states that clinicians should not use imaging studies as the initial test in patients who have low or intermediate pretest probability of PE 1.
The Diagnostic Algorithm
- If D-dimer is negative (using age-adjusted cutoff of age × 10 ng/mL for patients over 50 years): PE is ruled out, and no imaging is needed 1
- If D-dimer is positive: Proceed to CT pulmonary angiography (CTPA) 1, 2
The age-adjusted D-dimer threshold maintains sensitivity above 97% while significantly increasing specificity compared to the generic 500 ng/mL cutoff 1.
Why Other Options Are Incorrect
V/Q scan is reserved for patients who have a contraindication to CTPA or when CTPA is not available—it is not a first-line test 1, 2.
CTPA (CCTA chest) should not be ordered as the initial test in low-to-intermediate probability patients because it exposes them to unnecessary radiation and contrast when D-dimer could safely exclude PE in approximately 30% of cases 1, 3. CTPA is the appropriate first-line imaging test only in patients with high pretest probability of PE 1.
"No further testing" is incorrect because this patient does not meet all 8 Pulmonary Embolism Rule-Out Criteria (PERC), which would be required to safely forgo all testing in a low-risk patient 1, 2. PERC criteria include age <50, heart rate <100, oxygen saturation ≥95%, no hemoptysis, no estrogen use, no prior DVT/PE, no unilateral leg swelling, and no recent surgery/trauma 1.
Critical Pitfall to Avoid
Do not skip D-dimer testing and proceed directly to CTPA in this patient. This is a common error that leads to overutilization of CT imaging, increased radiation exposure, contrast-related complications, and detection of clinically insignificant subsegmental emboli that may lead to unnecessary anticoagulation 1. Studies demonstrate that using D-dimer first can safely avoid imaging in 30-32% of patients with suspected PE 1, 3, 4.
Special Consideration for CAD History
While this patient's history of CAD and CABG raises concern for acute coronary syndrome as an alternative diagnosis, the presence of a possible alternative diagnosis does not eliminate the need to evaluate for PE when it remains in the differential 1, 5. The normal EKG without ST changes makes acute MI less likely but does not exclude PE. The D-dimer test will help risk-stratify for PE while you simultaneously evaluate for cardiac causes 5, 6.