Pulmonary Embolism Rule-out Criteria

The Pulmonary Embolism Rule-out Criteria (PERC Rule) is a clinical decision-making tool designed to aid Emergency Physicians in the disposition of patients in whom pulmonary embolism is suspected, but unlikely. Unlike the Wells Score and Geneva Score, which are clinical prediction rules intended to risk stratify patients with suspected PE, the PERC Rule is designed to rule-out risk of PE in patients when the physician has already stratified them into a low-risk category.
Patients in this low risk category without any of these criteria may undergo no further diagnostic testing for Pulmonary Embolism. The rationale behind this decision is that the risk of further testing (specifically CT angiogram of the chest) have the potential to cause more harm (radiation exposure and risks from contrast dye) than the risk of PE.
*Hypoxia - Sa0<sub>2</sub> <95%
*Unilateral Leg Swelling
*Hemoptysis
*Prior DVT or PE
*Recent Surgery or Trauma
*Age >50
*Hormone Use
*TachycardiaThe Evidence:
A large randomized controlled trial recently validated the PERC rule which was derived in 2004. This trial enrolled over 8000 patients from 12 EDs in the US and 1 in New Zealand including both urban and community based centers. Patients were eligible if the ED MD ordered a diagnostic test to exclude PE. Physicians were also asked to fill out a standardized data collection form to include their clinical probability for PE (low, moderate, or high risk) before test results were obtained. All patients were followed up at 45 days to determine if they had suffered a PE. In patients that were considered low risk for PE by clinician gestalt (1666 patients), application of the PERC rule would have safely eliminated diagnostic testing in 99%. In other words, applying the PERC criteria to a low risk population of ED patients in whom PE is suspected would result in missing the diagnosis only 1% of the time.
The diagnostic characteristics of the PERC rule are summarized in Table 2 (below).
TABLE 2 Result 95% Confidence
Interval
Sensitivity 97.4% 95.8% to 98.5%
Specificity 21.9% 21.0% to 22.9%
Likelihood Ratio 0.12 0.07 to 0.19
Negative
The Caveats:
Deciding your comfort level with missing a particular disease is important in how decision aids such as the PERC rule are interpreted and implemented. Most would argue that a miss rate of 0% is neither attainable nor reasonable. For disorders that carry significant morbidity and mortality, many physicians would consider a miss rate of 2% or less to be acceptable. While the PERC rule meets this cut off, be sure that your definition of low risk mirrors that of the practicing MDs in this trial.The Outcome: After reviewing the evidence, you are relieved to note that your patient is entirely PERC negative. Combined with your initial clinical gestalt the PERC rule supports your decision to not pursue an evaluation for PE. Two months later, your decision is validated when the patient returns for an unrelated minor injury and has remained healthy
Validation
The PERC Rule was validated in a prospective multi-center study. The PERC Rule was shown to have a sensitivity of 97.4% and specificity of 21.9% with a false negative rate of 1.0% (16/1666).

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