What is the PERC (Pulmonary Embolism Rule-out Criteria) rule for determining the need for a computed tomography (CT) scan in patients with suspected pulmonary embolism (PE)?

From the Guidelines

The PERC rule is a clinical decision tool used to identify patients at very low risk of pulmonary embolism who don't need further testing with CT scans, and it should only be applied to patients with a low pre-test probability of PE based on clinical judgment, not those with moderate or high suspicion of PE. To apply the PERC rule, a patient must meet all eight criteria: age less than 50 years, heart rate less than 100 beats per minute, oxygen saturation greater than or equal to 95% on room air, no unilateral leg swelling, no hemoptysis, no recent trauma or surgery (within 4 weeks), no history of venous thromboembolism, and no hormone use (estrogen-containing contraceptives or hormone replacement therapy) 1. If a patient meets all these criteria and has a low clinical suspicion for PE, the probability of PE is less than 2%, making it safe to rule out PE without additional testing, as the likelihood of PE is 0.3% and no further testing is required 1.

Key Points to Consider

• The PERC rule helps reduce unnecessary radiation exposure, contrast-related complications, and healthcare costs by avoiding d-dimer testing in low-risk patients, which can lead to false-positive results and subsequent unnecessary CT scans 1.
• The PERC rule should not be applied to patients at intermediate or high risk for PE, and diagnostic testing strategy for these patients should begin with d-dimer testing, using age-adjusted d-dimer thresholds to determine whether imaging is warranted 1.
• The use of age-adjusted d-dimer thresholds (age × 10 ng/mL rather than a generic 500 ng/mL) in patients older than 50 years can help maintain sensitivity and increase specificity for the diagnosis of acute PE 1.

Clinical Application

• Clinicians should use validated clinical prediction rules, such as the PERC rule, to estimate pretest probability in patients in whom acute PE is being considered, and avoid obtaining d-dimer measurements or imaging studies in patients with a low pretest probability of PE who meet all Pulmonary Embolism Rule-Out Criteria 1.
• Clinicians should obtain a high-sensitivity d-dimer measurement as the initial diagnostic test in patients who have an intermediate pretest probability of PE or in patients with low pretest probability of PE who do not meet all Pulmonary Embolism Rule-Out Criteria, and use age-adjusted d-dimer thresholds to determine whether imaging is warranted 1.

From the Research

Overview of the PERC Rule

• The Pulmonary Embolism Rule-out Criteria (PERC) rule is a decision-making tool used to determine the need for a computed tomography (CT) scan in patients with suspected pulmonary embolism (PE) 2, 3.
• The PERC rule consists of nine components, and if all components are negative, it can be used to exclude PE without the need for further diagnostic testing 3.

Application of the PERC Rule

• Studies have shown that the PERC rule can safely replace D-dimer testing in patients suspected of PE, but deemed 'PE unlikely' by the dichotomized Wells score 2.
• However, the PERC rule may still miss around 8% of confirmed PE in patients who are deemed 'PE unlikely' by a dichotomized Wells score, and caution is advised in using the PERC rule as a substitute for the standard D-dimer test in all these patients 2.
• The use of the PERC rule has been shown to reduce the number of unnecessary CT pulmonary angiograms (CTPA) ordered in emergency departments 4, 5.

Diagnostic Performance of the PERC Rule

• The sensitivity of the PERC rule for detecting PE has been reported to be around 91.4%, with a negative likelihood ratio of 0.25 and a negative predictive value of 99.1% 2.
• In comparison, the sensitivity of the standard D-dimer test has been reported to be around 97.1%, with a negative likelihood ratio of 0.04 and a negative predictive value of 99.8% 2.

Clinical Implications

• The PERC rule can be a useful tool in reducing the number of unnecessary CTPA ordered in emergency departments, but it should be used in conjunction with clinical judgment and other diagnostic tools 5.
• The use of age-adjusted D-dimer and the PERC rule can also help reduce the number of indications for CTPA in patients with suspected PE 4.