What is the best initial test to diagnose pulmonary embolism in this patient?

Diagnostic Approach for Suspected Pulmonary Embolism

Plasma D-dimer should be ordered as the initial test to help rule out pulmonary embolism in this patient with intermediate pretest probability. 1, 2

Assessment of Pretest Probability

This 64-year-old man presents with acute dyspnea and has several clinical features that require evaluation for possible pulmonary embolism (PE):

• Age 64 years
• Acute dyspnea for the past hour
• History of coronary artery bypass graft surgery 5 years ago
• Tachycardia (heart rate 95 bpm)
• Tachypnea (respiratory rate 22)
• Hypertension (BP 142/96)

Using validated clinical prediction rules as recommended by the American College of Physicians 1, this patient would be classified as having an intermediate pretest probability for PE based on:

• Age > 50 years
• Recent onset of symptoms
• Tachycardia (nearly 100 bpm)
• Tachypnea

The patient does not meet PERC (Pulmonary Embolism Rule-Out Criteria) due to his age >50 years and elevated heart rate, so further testing is required 2.

Diagnostic Algorithm

For patients with intermediate pretest probability of PE, the diagnostic approach should follow these steps:

1. Obtain a high-sensitivity D-dimer measurement first 1, 2

   • If D-dimer is negative (below age-adjusted cutoff), PE can be reliably excluded
   • For this 64-year-old patient, the age-adjusted D-dimer threshold would be 640 ng/mL (age × 10 ng/mL) 1, 2

2. If D-dimer is positive (above age-adjusted cutoff):

   • Proceed to CT pulmonary angiography (CTPA) 1, 2, 3

Why D-dimer First?

The American College of Physicians explicitly recommends against using imaging studies as the initial test in patients with low or intermediate pretest probability 1. Starting with D-dimer in intermediate-risk patients:

• Avoids unnecessary radiation exposure if negative
• Is cost-effective
• Has high sensitivity (>95%) for ruling out PE when negative
• Follows evidence-based guidelines 1, 2

Important Considerations

• Age-adjusted D-dimer thresholds (age × 10 ng/mL) should be used for patients >50 years to improve specificity without sacrificing sensitivity 1, 2
• If D-dimer is positive, CTPA is the recommended imaging modality for definitive diagnosis 1, 3, 4
• Ventilation-perfusion (V/Q) scanning should be reserved for patients with contraindications to CTPA (such as renal insufficiency or contrast allergy) 1, 5

Common Pitfalls to Avoid

• Proceeding directly to imaging without D-dimer testing in intermediate-risk patients increases radiation exposure and costs 1, 2
• Using non-age-adjusted D-dimer cutoffs in older patients reduces specificity and leads to unnecessary imaging 1, 2
• Ordering D-dimer in high-probability patients (not applicable to this case) can lead to false negatives 2

By following this evidence-based approach, you can efficiently and safely evaluate this patient for possible pulmonary embolism while minimizing unnecessary testing and radiation exposure.