PERC Rule for Pulmonary Embolism

The PERC rule is a "rule-out" tool for low-risk patients (probability of ≤ 15% for PE) where the diagnosis of pulmonary embolism (PE) is being considered. PERC negative (criteria) typically allows the clinician to avoid further testing, such as a d-dimer which has a high sensitivity but low specificity. 

The PERC Rule is not meant to diagnose Pulmonary Embolism (PE). Typically, a patient is deemed low-risk by clinician's gestalt. If the patient is moderate or high risk then the PERC cannot be used. However, there are stratification tools, such as the Wells' Criteria, that may be used to estimate pre-test probability required to guide workup when considering a PE diagnosis. In clinical practice, the Well's score is often used to predict who is low-risk and then apply the PERC rule to stop workup for PE. 

It is important to note that a d-dimer can be falsely positive for many patients with non-VTE. Following an order of operations in the patient being considered for a PE is vital to good patient outcomes and minimizing harm.

• There must first be clinical suspicion for PE in the patient, such as presentation of symptoms (chest pain, shortness of breath, etc.). 
• A thorough history and physical must be completed first.
• Second to a history and physical is application of the PERC Rule to help avoid further testing in a low risk patient. 
• A common mistake made is applying the PERC Rule or ordering the d-dimer before the history and physical. For example, by applying the rule or ordering a d-dimer on all patients with chest pain or shortness of breath or to all patients with leg pain or swelling without first obtaining a history and physical exam. 

Criteria

If any of the following criteria is/are present, the PERC rule is not satisfied and cannot be used to rule out PE in this patient.

Results

No need for further workup: < 2% chance of PE

• If no criteria are positive and clinician's pre-test probability is <15%, PERC Rule criteria are satisfied.

Cannot Rule-out PE

If any criteria are positive, the PERC rule cannot be used to rule out PE in this patient.

• In the context of a low-risk patient who is not PERC negative, the clinician ought to consider a d-dimer for further evaluation.
• If the d-dimer is negative in the context of a low-risk patient with a pre-test probability of > 15%, then the patient does not require further testing for PE.
• If the d-dimer is positive, further testing is indicated, such as a CTA.
• Consider using the Wells' Criteria for risk stratification and workup guidance, or obtain a d-dimer or imaging based on risk.

The PERC Rule is a "rule-out" tool and is not meant for risk stratification.

• To use this rule, the clinician must have a gestalt that the patient's risk is low (< 15%).
• If the patient is moderate or high risk then the PERC cannot be used. Consider using the Wells' Criteria for risk stratification and workup guidance, or obtain a d-dimer or imaging based on risk.

A d-dimer can be falsely positive for many patients with non-VTE*.

• There must first be clinical suspicion for PE in the patient, such as presentation of symptoms (chest pain, shortness of breath, etc.). 
• A thorough history and physical must be completed first.
• Second to a history and physical is application of the PERC Rule to help avoid further testing in a low risk patient. 
• A common mistake made is applying the PERC Rule or ordering the d-dimer before the history and physical. For example, by applying the rule or ordering a d-dimer on all patients with chest pain or shortness of breath or to all patients with leg pain or swelling without first obtaining a history and physical exam. 

*Venous Thromboembolism (VTE)