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In the fast-paced environment of emergency medicine, clinicians frequently face a significant challenge: distinguishing between common symptoms like chest pain or shortness of breath and a life-threatening pulmonary embolism (PE). While PE requires prompt diagnosis and treatment, over-testing can expose patients to unnecessary radiation, contrast dye risks, and significant healthcare costs. The PERC Calculator provides a simple, evidence-based solution, empowering healthcare professionals to safely rule out PE in a select group of low-risk patients without the need for further investigation.
This comprehensive guide explains the Pulmonary Embolism Rule-out Criteria (PERC), details how to use this effective PE risk assessment tool, and clarifies its role within the broader pulmonary embolism diagnostic pathway.
The PERC rule is a clinical decision support tool designed to identify patients with a very low pre-test probability of pulmonary embolism. It’s crucial to understand that it is not a diagnostic test; rather, it is a checklist to determine who is so unlikely to have a PE that further testing is not warranted. Developed by Dr. Jeffrey Kline and colleagues, its purpose is to reduce diagnostic imaging in the emergency department.
The strength of the PERC rule lies in its high sensitivity and negative predictive value. When a low-risk patient meets all eight criteria (is “PERC negative”), the probability of them having a PE is less than 2%, a risk level considered acceptable to forego further testing.
To be considered “PERC negative,” a patient must meet all of the following eight criteria:
Using the PERC calculator is a straightforward process designed for rapid clinical assessment. The most critical first step, however, happens before you even apply the rule.
The PERC rule is only validated for patients whom the clinician, based on their experience and judgment (“gestalt”), already considers to have a low probability of having a pulmonary embolism. If you have a moderate or high suspicion for PE, you should skip PERC and proceed directly to other risk stratification tools like the Wells’ Score or imaging.
Once you’ve established the patient is low-risk, systematically go through the eight criteria. For each of the questions below, a “No” answer is required to satisfy the rule.
The outcome is binary and provides clear direction for the next steps in your emergency PE evaluation.
The power of the PERC calculator comes from its clear, actionable results. Correctly interpreting a “PERC negative” or “PERC positive” finding is key to safe and efficient patient care.
A PERC negative result is achieved when the patient meets all eight criteria (answering “No” to all questions). This outcome has a very important meaning:
This is the primary benefit of the PERC rule—it safely concludes the PE workup, avoiding the risks and costs of additional testing.
A PERC positive result occurs if even one of the eight criteria is not met (one or more “Yes” answers). It is critical to understand what this means:
Clinicians often use both the PERC rule and the Wells’ Score, but they serve different purposes at different stages of the diagnostic process. The PERC rule is a “rule-out” tool for the lowest-risk group, while the Wells’ Score is a broader risk stratification tool.
| Feature | PERC Rule | Wells’ Score for PE |
|---|---|---|
| Primary Purpose | To identify patients who need NO further PE testing. | To stratify patients into low, moderate, or high-risk categories to guide further testing (D-dimer or imaging). |
| Target Population | Only patients with a very low clinical pre-test probability. | Any patient in whom PE is being considered. |
| Outcome | Binary: PERC negative (stop) or PERC positive (continue workup). | A score that categorizes risk (e.g., PE unlikely vs. PE likely). |
| Next Step if “Positive” | Proceed to Wells’ Score and/or D-dimer testing. | Proceed to D-dimer testing (if low/moderate risk) or direct to imaging (if high risk). |
The most effective pulmonary embolism diagnostic pathway often uses these tools sequentially:
This staged approach ensures that only patients who truly need it are exposed to further testing, making it a cornerstone of modern low-risk PE exclusion strategies.
While the PERC rule is an excellent tool, its safety depends on its correct application. It should NOT be used in the following situations:
A PERC negative result means the patient has a post-test probability of PE of less than 2%. This is widely accepted as a safe threshold to stop the diagnostic workup for PE and investigate other potential causes for their symptoms without ordering a D-dimer or CT scan.
The PERC rule should never be applied if your initial clinical assessment is that the patient has a moderate or high probability of having a PE. It is exclusively a tool for the “low-risk” population. It is also not validated for pregnant or inpatient populations.
PERC acts as a preliminary screening tool. It is used first in low-risk patients. If the patient is PERC negative, the workup stops. If they are PERC positive, the Wells’ score is then used as the next step to stratify their risk and determine whether a D-dimer test or direct imaging is the most appropriate next step.
The eight criteria are: age under 50, heart rate under 100 bpm, oxygen saturation 95% or higher, no unilateral leg swelling, no hemoptysis, no recent surgery/trauma, no prior VTE (DVT or PE), and no estrogen use.
The oxygen saturation criterion specifically requires a value of ≥95% on room air. Therefore, a patient on supplemental oxygen cannot be assessed with this rule. A history of cancer is a major risk factor for PE and generally moves a patient out of the “low-risk” category, making the PERC rule inappropriate for them.
The PERC calculator is an integral first step in an evidence-based diagnostic algorithm for suspected PE. Understanding where it fits is essential for optimal patient care.
The PERC calculator is an indispensable PE risk assessment tool for any clinician working in an acute care setting. By correctly identifying low-risk patients who do not require further testing, it helps reduce patient harm from unnecessary radiation and contrast, decreases emergency department length of stay, and lowers healthcare costs. Mastering the pulmonary embolism rule-out criteria and understanding when to apply them is a simple and effective way to improve your clinical practice.
Source: MDCalc — mdcalc.com
Use the PERC (Pulmonary Embolism Rule-out Criteria) score to assess if a patient with a low pre-test probability of PE can be safely ruled out without further testing.