The Pulmonary Embolism Rule-out Criteria (PERC) is a validated clinical decision rule designed to identify patients with a very low probability of pulmonary embolism (PE) who may not require further diagnostic testing. When applied appropriately in carefully selected patients, the PERC Rule can safely reduce unnecessary D-dimer testing, exposure to diagnostic imaging, and associated healthcare costs while maintaining a very low risk of missed pulmonary embolism.
Unlike diagnostic scoring systems that estimate clinical probability, the PERC Rule is intended to determine whether pulmonary embolism can be reasonably excluded without further investigation in patients who have already been assessed as being at low risk.
Purpose of the PERC Rule
Pulmonary embolism frequently presents with non-specific symptoms such as dyspnoea, pleuritic chest pain, tachycardia, syncope, or unexplained hypoxaemia. Because these symptoms overlap with numerous cardiopulmonary conditions, indiscriminate investigation may result in unnecessary laboratory testing, radiation exposure, intravenous contrast administration, and increased healthcare resource utilisation.
The PERC Rule provides a structured method of identifying patients in whom the likelihood of pulmonary embolism is sufficiently low that further diagnostic testing is unlikely to provide clinical benefit.
Importantly, the PERC Rule is not a diagnostic test and does not estimate pre-test probability. It should only be applied after an experienced clinician has already determined that the patient has a low clinical probability of pulmonary embolism.
PERC Eligibility Criteria
To satisfy the Pulmonary Embolism Rule-out Criteria, all eight criteria must be negative.
The patient must have:
- Age under 50 years
- Heart rate below 100 beats per minute
- Oxygen saturation 95% or greater on room air
- No unilateral leg swelling
- No haemoptysis
- No recent surgery or trauma requiring hospitalisation
- No previous deep vein thrombosis (DVT) or pulmonary embolism (PE)
- No hormone use, including oestrogen-containing therapy
If any one of these criteria is present, the PERC Rule is considered positive, and pulmonary embolism cannot be excluded using the rule alone. Further assessment should then proceed according to current diagnostic pathways.
Appropriate Use
The PERC Rule should only be applied in patients with an already established low pre-test probability of pulmonary embolism.
Clinical probability is commonly estimated using:
- Experienced clinical judgement
- The Wells Score for Pulmonary Embolism
- Other validated clinical prediction models where appropriate
Patients with moderate or high clinical suspicion should proceed directly to appropriate diagnostic investigation rather than relying on the PERC Rule.
Role in Diagnostic Assessment
The PERC Rule forms part of an evidence-based diagnostic strategy for suspected pulmonary embolism.
Typical assessment includes:
- Clinical history and physical examination.
- Estimation of pre-test probability.
- Application of the PERC Rule in low-risk patients.
- D-dimer testing when indicated.
- Diagnostic imaging, such as computed tomography pulmonary angiography (CTPA) or ventilation-perfusion (V/Q) scanning, where appropriate.
When all PERC criteria are satisfied in a patient with low pre-test probability, pulmonary embolism may often be excluded without D-dimer testing or imaging, thereby avoiding unnecessary investigations and their associated risks.
Clinical Applications
The PERC Rule is widely used across multiple healthcare settings, including:
- Emergency departments
- Acute medical units
- Urgent care centres
- Emergency medicine services
- Selected primary care settings
- Observation units
Its incorporation into numerous emergency medicine protocols reflects strong evidence supporting structured assessment of low-risk patients with suspected pulmonary embolism.
Limitations
Although the PERC Rule has been extensively validated, it should not be applied indiscriminately or used as a substitute for clinical judgement.
Caution is required in patients with:
- Moderate or high clinical suspicion of pulmonary embolism
- Pregnancy
- Significant cardiopulmonary disease
- Persistent or unexplained symptoms
- Haemodynamic instability
- Situations where clinical assessment suggests an alternative diagnostic pathway
The PERC Rule should always be interpreted within the broader clinical context, and experienced clinical judgement remains the most important determinant of patient management.
PERC Rule in Clinical Practice
The PERC Rule is commonly used alongside other validated assessment tools, including the Wells Score for Pulmonary Embolism, high-sensitivity D-dimer testing, and diagnostic imaging pathways recommended by organisations such as the National Institute for Health and Care Excellence (NICE), the European Society of Cardiology (ESC), and the American College of Emergency Physicians (ACEP). Together, these approaches provide a structured, evidence-based framework for evaluating suspected venous thromboembolism.
Clinical Significance
The Pulmonary Embolism Rule-out Criteria remains one of the most important clinical decision rules for evaluating patients with suspected pulmonary embolism. When applied appropriately in patients with a genuinely low pre-test probability, it enables clinicians to safely exclude pulmonary embolism without unnecessary laboratory testing or imaging. Used alongside comprehensive clinical assessment, validated prediction tools, and current guideline recommendations, the PERC Rule supports safe, efficient, and evidence-based management of suspected pulmonary embolism.