Free PERC Calculator - Pulmonary Embolism Rule-Out Criteria

What Is the PERC Rule?

The Pulmonary Embolism Rule‑Out Criteria (PERC) is a clinical decision tool developed for emergency departments to help physicians rapidly evaluate patients with suspected pulmonary embolism (PE). Originally introduced in a 2005 study and validated two years later, the PERC rule consists of eight simple yes/no questions. When all answers are negative, the likelihood of PE is considered low enough to safely skip D‑dimer testing and further imaging, making it a convenient PE screening tool. This PERC calculator automates the scoring process, allowing you to check each criterion quickly and obtain a result along with a brief interpretation.

When to Use the PERC Score

The PERC rule is most valuable as a supplement to the Wells score. It is intended for patients in whom the clinical probability of PE is already low to moderate. According to common practice, if the Wells score is ≤4 (or ≤1 in the simplified version) and the patient meets the age‑based pre‑test criteria, the PERC rule can be applied. When the rule is negative, D‑dimer testing can be avoided, reducing unnecessary laboratory work and radiation exposure. The tool is not designed for patients with a high pre‑test probability, because the negative predictive value may not be sufficient.

How to Use the PERC Calculator

Using this PERC rule calculator is straightforward:

1. Confirm that the patient is appropriate for the PERC rule (see limitations below).
2. Review each of the eight criteria and mark those that are present (positive). All criteria start unchecked, so you only check the ones that apply.
3. The calculator automatically tallies the results and displays whether the PERC score is negative or positive, accompanied by a short comment.

No complex inputs are required—the tool focuses purely on the eight clinical factors.

The Eight PERC Criteria

Each criterion represents a clinical finding or historical element that increases the suspicion for PE. If none of the following conditions are present, the PERC score is negative.

• Age ≥ 50 years — The patient is 50 years old or older.
• Heart rate > 100 beats per minute — Tachycardia is defined as a resting heart rate exceeding 100 bpm. Note that concurrent use of β‑blockers can mask tachycardia.
• O₂ saturation < 95% on room air — Hypoxemia while breathing ambient air. This criterion should not be used for patients who require supplemental oxygen.
• Unilateral leg swelling — Swelling in one leg, especially if accompanied by pain or recent onset, may indicate deep venous thrombosis (DVT).
• Hemoptysis — The patient is coughing up blood.
• Prior DVT or PE — A documented history of deep venous thrombosis or pulmonary embolism suggests a predisposition to thromboembolic events.
• Recent surgery or trauma — Any major surgery or significant trauma that occurred within the past 4 weeks qualifies as a risk factor.
• Exogenous estrogen use — The patient is taking estrogen‑containing medication, such as oral contraceptives, hormone replacement therapy, or other estrogen‑based treatments.

Interpreting the Results

The PERC score yields only two possible outcomes:

• PERC negative — None of the eight criteria are present. In this case, the risk of PE is very low (historically <15% in the derivation cohort), and the patient can safely avoid D‑dimer testing.
• PERC positive — At least one criterion is present. Further evaluation is warranted, typically starting with a D‑dimer assay and, if indicated, imaging such as CT pulmonary angiography.

It is important to remember that the PERC rule is a screening tool, not a diagnostic test. A negative result does not absolutely rule out PE, but it brings the probability low enough that the benefits of testing are outweighed by the risks.

Limitations of the PERC Rule

The PERC criteria were derived and validated in emergency department populations without certain coexisting conditions. Therefore, the rule may be unreliable for patients who:

• Have active cancer
• Have known thrombophilia or a strong family history of thrombophilia
• Are taking β‑blockers, which can mask tachycardia
• Present with transient tachycardia
• Have a leg amputation
• Are severely obese (BMI ≥ 30), which may obscure leg swelling
• Have long‑term hypoxemia <95% at baseline

When any of these conditions apply, the PERC score should not be used as the sole decision‑making tool. Clinical judgment remains essential.

This calculator is designed to assist with rapid screening but cannot replace a comprehensive assessment by a qualified healthcare professional. Always consult a physician if you have concerns about a patient’s condition.