What is a Pulmonary Embolism Rule-out Criteria (PERC) score and what are its determinants?

Pulmonary Embolism Rule-Out Criteria (PERC) Score

The PERC score is a validated clinical decision tool designed to identify patients at very low risk for pulmonary embolism (PE) in whom further diagnostic testing can be safely avoided. 1

PERC Score Determinants

The PERC score consists of 8 clinical criteria that must ALL be negative to safely rule out PE without further testing:

1. Age < 50 years
2. Heart rate < 100 beats/minute
3. Oxygen saturation ≥ 95% (at sea level)
4. No unilateral leg swelling
5. No hemoptysis
6. No recent trauma or surgery (within 4 weeks)
7. No history of venous thromboembolism (VTE)
8. No hormone use (estrogen) 1, 2

Clinical Application

When to Apply PERC

• PERC should ONLY be applied to patients with a low clinical probability of PE based on clinical gestalt or another validated tool 1, 2
• It is NOT a screening tool for all patients, but rather for those in whom PE is being considered but deemed unlikely 1

Interpretation

• If ALL 8 criteria are negative (PERC negative): No further testing is needed as the risk of PE is lower than the risks of testing (approximately 1%) 1, 2
• If ANY criterion is positive (PERC positive): Proceed with D-dimer testing 1

Evidence and Performance

• Meta-analysis of 12 studies showed PERC has a sensitivity of 97% and specificity of 22% for PE 1
• The missed PE rate using PERC is only 0.3% (44 of 14,844 cases) 1
• Using PERC could safely avoid approximately 22% of D-dimer tests in appropriate populations 1

Common Pitfalls and Caveats

1. Inappropriate application: PERC should ONLY be applied to patients with low pretest probability of PE, not all patients with suspected PE 1

2. High prevalence populations: Some studies suggest caution when using PERC in populations with high PE prevalence. A European study found a 5.4% PE rate in PERC-negative patients overall 3, though a later European study found only 1.2% PE rate in PERC-negative patients with low clinical probability 4

3. Clinical judgment remains important: When PERC is combined with clinical gestalt assessment of low probability, it performs better than when combined with formal scoring systems like the revised Geneva score 5

4. Testing threshold: Decision analysis suggests the testing threshold for PE is approximately 1.4%, meaning patients with a pretest probability below this threshold should not undergo testing 6

Diagnostic Algorithm for Suspected PE

1. Assess clinical probability of PE using gestalt or validated tool
2. For LOW probability patients:
   • Apply PERC criteria
   • If PERC negative (all 8 criteria met): No further testing needed
   • If PERC positive (any criterion not met): Order D-dimer test
3. For INTERMEDIATE probability patients:
   • Order D-dimer test
   • If D-dimer normal (ideally age-adjusted): No further testing
   • If D-dimer elevated: Proceed to imaging
4. For HIGH probability patients:
   • Proceed directly to imaging (CTPA)
   • Do NOT rely on D-dimer 1, 2

The PERC rule represents a significant advance in avoiding unnecessary testing in very low-risk patients, with potential to reduce healthcare costs, radiation exposure, and complications from unnecessary testing while maintaining patient safety 7.