Medical Diagnostics & Clinical Scoring

PERC Rule for Pulmonary Embolism

Apply the Pulmonary Embolism Rule-out Criteria (PERC) to safely avoid D-dimer testing and imaging in low-risk emergency patients.

PERC Rule Negative

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The Pulmonary Embolism Rule-out Criteria (PERC) is an 8-point clinical decision rule designed to identify patients in whom the suspicion of pulmonary embolism is so low that diagnostic testing (like D-dimer or CT pulmonary angiography) will do more harm than good.

The Problem with Over-Testing

Testing for PE has increased dramatically, largely due to the availability of CT angiography. However, testing carries risks: false-positive D-dimers lead to unnecessary radiation exposure from CTs, contrast-induced nephropathy, and the over-diagnosis of sub-segmental PEs that may not require anticoagulation. PERC helps halt this cascade in very low-risk patients.

The 8 Criteria

A patient is PERC negative (and thus PE is ruled out clinically) only if they meet ALL of the following:

  1. Age < 50 years
  2. Heart rate < 100 beats per minute
  3. Room air oxygen saturation ≥ 95%
  4. No prior history of DVT or PE
  5. No recent trauma or surgery requiring hospitalization (within 4 weeks)
  6. No hemoptysis (coughing up blood)
  7. No exogenous estrogen use (e.g., oral contraceptives, hormone replacement)
  8. No unilateral leg swelling

PERC Negative = (Age < 50) AND (HR < 100) AND (O2 >= 95%) AND (No Hemoptysis) AND (No Estrogen) AND (No Prior DVT/PE) AND (No Surgery/Trauma) AND (No unilateral leg swelling).

Where:
PERC=
Pulmonary Embolism Rule-out Criteria
Low Pre-test Probability=
The rule can only be applied if the clinician gestalt already assesses the PE risk as low (<15%).

The Crucial Prerequisite: Pre-Test Probability

The most common mistake when using PERC is applying it to the wrong patient. PERC is a "step-down" rule. It must only be applied to patients whose pre-test probability is already assessed as low (often defined as <15% gestalt). If you think a patient is highly likely to have a PE, PERC cannot rule it out.

Frequently Asked Questions

The PERC rule should ONLY be applied to patients in whom the clinician's initial assessment yields a low pre-test probability of Pulmonary Embolism (typically defined as <15%). It is not for high or moderate-risk patients.

If a low-risk patient is negative for all 8 PERC criteria, the probability of PE drops to less than 2%. The clinician can safely rule out PE without drawing a D-dimer test or ordering a CT scan.

If a patient meets even one criterion (PERC positive), they cannot be ruled out clinically. The standard next step is usually drawing a D-dimer test.

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