PERC Calculator

What Is the PERC Rule?

The Pulmonary Embolism Rule-out Criteria (PERC) rule is a clinically validated clinical decision rule designed to identify emergency department and acute-care patients who have a very low pre-test probability of acute pulmonary embolism (PE). When a patient meets all eight criteria of the PERC rule (PERC negative), the probability of acute pulmonary embolism is less than 1.8%. In this group, further diagnostic testing such as D-dimer blood assays or computed tomography pulmonary angiography (CTPA) can be safely avoided without compromising patient safety.

By eliminating unnecessary imaging, the PERC rule reduces patient exposure to ionizing radiation, prevents intravenous contrast-induced complications, shortens hospital lengths of stay, and minimizes healthcare costs. For related clinical calculators, explore our Target Heart Rate Calculator and Heart Disease Risk Calculator.

The 8 Criteria of the PERC Rule

To qualify as PERC negative, a patient must satisfy every single one of the following eight criteria (no risk factors present):

• Age < 50 years: Advanced age increases the baseline incidence of venous thromboembolism and naturally raises D-dimer levels.
• Heart Rate < 100 bpm: Resting sinus tachycardia is one of the most common early physiological responses to pulmonary arterial occlusion and right ventricular strain.
• Pulse Oximetry SpO2 ≥ 95% on Room Air: Normal arterial oxygen saturation helps rule out significant ventilation-perfusion mismatch.
• No Prior History of DVT or PE: A documented history of deep vein thrombosis or pulmonary embolism places the patient in a higher baseline recurrence risk category.
• No Recent Surgery or Major Trauma in the Past 4 Weeks: Recent tissue injury or surgical procedures requiring hospitalization induce a temporary hypercoagulable state.
• No Hemoptysis: Coughing up blood suggests pulmonary infarction or alveolar hemorrhage and warrants diagnostic imaging.
• No Exogenous Estrogen Use: Combined oral contraceptive pills, hormone replacement therapy (HRT), and estrogenic compounds elevate hepatic synthesis of clotting factors.
• No Unilateral Leg Swelling: Asymmetrical lower extremity swelling or tenderness strongly indicates acute deep vein thrombosis, which is the primary source of pulmonary emboli.

How to Interpret PERC Results

The clinical interpretation depends on whether the patient meets all criteria:

• PERC Negative (Score = 0): All 8 criteria are met (0 positive risk factors). In a patient with low pre-test clinical suspicion (Wells score < 2 or clinician gestalt < 15%), pulmonary embolism is ruled out. No D-dimer test or radiographic imaging is required.
• PERC Positive (Score ≥ 1): One or more criteria are failed. The PERC rule cannot rule out PE. The clinician must proceed to the next diagnostic step, typically starting with a high-sensitivity D-dimer test or direct imaging if clinical suspicion is moderate or high.

Mathematical Basis of the Rule-Out Threshold

The PERC rule relies on Bayes' theorem to lower the post-test probability of PE below the test threshold (the point at which the risk of diagnostic testing outweighs the risk of untreated disease):

With a negative likelihood ratio of approximately 0.17 in low-risk cohorts (pre-test probability < 15%), the resulting post-test probability drops below 1.8%, satisfying the accepted safety threshold established by the American College of Emergency Physicians (ACEP).