When diagnosing pulmonary embolism (PE), what is the preferred initial screening test between d-dimer (D-dimer) and computed tomography angiography (CTA)?

D-dimer versus CTA for Diagnosing Pulmonary Embolism

D-dimer should be used as the initial screening test for patients with low or intermediate pretest probability of pulmonary embolism (PE), while computed tomography angiography (CTA) should be the first-line test for patients with high pretest probability of PE. This approach optimizes patient outcomes by minimizing unnecessary radiation exposure while ensuring timely diagnosis of potentially fatal PE.

Clinical Probability Assessment First

Before ordering any test, clinicians should:

1. Use validated clinical prediction rules (Wells score, Geneva score) or clinical gestalt to estimate pretest probability of PE
2. Categorize patients into low, intermediate, or high pretest probability groups

Diagnostic Algorithm Based on Pretest Probability

Low Pretest Probability

• Apply Pulmonary Embolism Rule-Out Criteria (PERC)
• If all PERC criteria met: No further testing needed 1
• If PERC criteria not all met: Obtain D-dimer test
  • If D-dimer negative: PE excluded, no imaging needed 1
  • If D-dimer positive: Proceed to CTA 1

Intermediate Pretest Probability

• Obtain D-dimer test
  • If D-dimer negative: PE excluded, no imaging needed 1
  • If D-dimer positive: Proceed to CTA 1

High Pretest Probability

• Proceed directly to CTA without D-dimer testing 1
• D-dimer testing is not recommended as it will not change management even if negative 1

D-dimer Considerations

• Use age-adjusted D-dimer cutoffs for patients >50 years (age × 10 ng/mL) to maintain sensitivity while improving specificity 1
• Standard cutoff for patients ≤50 years is 500 ng/mL
• D-dimer has excellent negative predictive value (>99%) but poor positive predictive value (~30%) 2
• D-dimer may be falsely elevated in:
  • Cancer
  • Pregnancy
  • Hospitalized patients
  • Advanced age
  • Inflammation

CTA Considerations

• Multidetector CTA is the imaging method of choice for PE diagnosis 1
• Advantages:
  • Excellent accuracy with sensitivity of 83% and specificity of 96% 1
  • Low rate of inconclusive results (3-5%) 1
  • May provide alternative diagnoses if PE is excluded
  • Readily available in most centers
• Disadvantages:
  • Radiation exposure (3-10 mSv)
  • Contrast-related risks (allergy, nephropathy)
  • Significant radiation exposure to breast tissue in young women

Common Pitfalls to Avoid

1. Ordering CTA as the initial test in low/intermediate probability patients

   • Increases radiation exposure
   • May lead to incidental findings requiring further workup
   • Increases healthcare costs

2. Skipping D-dimer in non-high probability patients

   • Misses opportunity to safely rule out PE without radiation exposure
   • Increases unnecessary CTA scans

3. Ordering D-dimer in high probability patients

   • Will not change management even if negative
   • Delays definitive diagnosis

4. Reviewing D-dimer results before clinical assessment

   • May bias clinical probability assessment 3
   • Clinical probability should be determined before reviewing D-dimer results

5. Using point-of-care D-dimer tests without considering limitations

   • Lower sensitivity than laboratory-based tests (88% vs 95%) 1
   • Should only be used in low pretest probability patients

By following this evidence-based approach, clinicians can optimize the diagnosis of PE while minimizing unnecessary testing, radiation exposure, and healthcare costs, ultimately improving patient outcomes through timely and accurate diagnosis.