In a patient with low pre‑test probability for pulmonary embolism (e.g., Wells score ≤4), can the Pulmonary Embolism Rule‑Out Criteria be used to safely exclude PE, and what are the eight criteria?

PERC Rule for Pulmonary Embolism Exclusion

In patients with low pre-test probability for pulmonary embolism, the Pulmonary Embolism Rule-Out Criteria can safely exclude PE without further testing when all eight criteria are met, avoiding unnecessary D-dimer testing and imaging in approximately 20% of low-risk patients. 1

The Eight PERC Criteria

All eight of the following must be present (negative) to safely rule out PE:

• Age < 50 years 1
• Heart rate < 100 beats per minute 1
• Oxygen saturation ≥ 95% on room air 1
• No unilateral leg swelling 1
• No hemoptysis 1
• No recent trauma or surgery requiring hospitalization within 4 weeks 1
• No prior history of venous thromboembolism (VTE) 1
• No oral hormone use (estrogen therapy or oral contraceptives) 1

Proper Application of PERC

PERC should only be applied to patients already determined to have low clinical probability of PE through gestalt assessment or validated scoring systems (Wells score ≤4 or revised Geneva score indicating low probability). 1 The rule was specifically designed to avoid initiating any diagnostic workup—including D-dimer testing—in patients whose likelihood of PE is extremely low. 1

The diagnostic algorithm proceeds as follows:

• First, assess clinical probability using gestalt or a validated prediction rule (Wells or revised Geneva score) 1
• If low probability is established, apply all eight PERC criteria 1
• When all eight criteria are met (PERC-negative), PE is safely excluded without D-dimer or imaging 1
• If any single PERC criterion is not met (PERC-positive), proceed to high-sensitivity D-dimer testing 2

Safety and Performance Data

Prospective validation demonstrates that PERC combined with low clinical probability reduces the post-test probability of VTE to approximately 1.0% (95% CI: 0.6-1.6%), which meets the accepted safety threshold of <2% for ruling out PE. 3 This approach identifies roughly 20% of patients with suspected PE in whom further testing can be safely avoided. 3

The sensitivity of PERC when combined with low gestalt probability is 97.4% (95% CI: 95.8-98.5%), with a specificity of 21.9%. 3 A randomized non-inferiority management study further supported safe exclusion of PE using this combined approach. 1

Critical Limitations and Caveats

PERC cannot be applied to patients over 50 years of age, as age < 50 is one of the eight required criteria. 2 For patients > 50 years with low clinical probability, the appropriate strategy is age-adjusted D-dimer testing (age × 10 ng/mL cutoff) rather than PERC. 2

The safety of PERC has been questioned in populations with high PE prevalence (>20-30%). 4 One external validation study in a population with 21.3% PE prevalence found that 6.4% of PERC-negative patients with low clinical probability still had PE, exceeding the 2% safety threshold. 4 However, the 2019 ESC guidelines note that the low overall PE prevalence in the original validation studies supports the rule's use in typical emergency department populations. 1

PERC should never be used as a general screening tool or applied to patients with intermediate or high clinical probability. 2 Doing so will result in missed diagnoses and potential harm. The rule's performance depends entirely on proper patient selection within the low-probability category. 2

Alternative for Patients Over 50 Years

For patients > 50 years with low clinical probability who cannot use PERC, the equivalent safe exclusion strategy is a negative age-adjusted D-dimer (age × 10 ng/mL). 2 This maintains sensitivity > 97% while improving specificity compared to the standard 500 ng/mL cutoff. 2

Practical Implementation

When PERC is properly applied to low-probability patients and all eight criteria are met:

• No D-dimer testing is required 2
• No imaging studies are needed 2
• The patient can be safely discharged 5
• The risk of testing (radiation exposure, contrast reactions, incidental findings, anticoagulation complications from false-positives) exceeds the risk of missed PE 5

The key pitfall is ordering D-dimer before completing clinical probability assessment and PERC evaluation. 2 This sequence error leads to unnecessary testing and defeats the purpose of the rule. 2