When the PERC Rule Fails

Kline et al developed a clinical decision tool based on parameters that could be obtained from a brief initial assessment to reasonably exclude the diagnosis of pulmonary embolism (PE) without the use of D-dimer in order to prevent unnecessary cost and the use of medical resources. ¹ Many of us have used the Pulmonary Embolism Rule-out Criteria (PERC) rule by now, but we should be clear on what it includes. Are we using it appropriately?

PERC Rule Criteria

1. Age < 50 years
2. HR < 100 bpm
3. Room air oxygen saturation > 94%
4. No prior history of DVT or PE
5. No recent trauma or surgery
6. No hemoptysis
7. No exogenous estrogen
8. No clinical signs suggestive of DVT

If ALL criteria are met, then the patient can be called “PERC ruled out” or “PERC negative.”

What are the performance characteristics of the PERC rule?

A review and meta-analysis published in Annals of EM in 2012 found 12 qualifying studies evaluating the PERC rule and ultimately determined the following ² :

• The pooled sensitivity of the PERC rule is 97.2%.
• The pooled negative LR was 0.17.
• The overall proportion of missed PEs was 0.32% (44 of 13,855 total cases).

When does the PERC rule fail?

So who are the CT-PE or V/Q positive patients who could have been falsely “PERC ruled out?” Is there data on false PERC rule negative patients as determined by a positive CT-PE or V/Q scan?

Kline et al reworked the data from a previous paper showing the outcomes of patients who presented to the ED and were diagnosed with PE ³ and used it as a dataset to retrospectively determine the characteristics of known PE patients who had zero of 8 PERC criteria. Out of 1,880 patients with PE, 114 were PERC-negative if it would have been applied. ⁴ Of these patients, they found only 3 variables demonstrating a significant difference in proportions between the PERC-negative and PERC-positive groups. These variables were:

1. Pleuritic chest pain (56% of 114 PERC negative patients vs 38% of 1,766 PERC positive patients)
2. Pregnancy (4% of 114 PERC negative patients vs 1% of 1,766 PERC positive patients)
3. Postpartum status (4% of 114 PERC negative patients vs 1% of 1,766 PERC positive patients)

PE and pleuritic chest pain

Pleuritic chest pain may be more predictive of PE than you think, as noted in the above statistics for false PERC negative cases. Variables commonly believed to modify the pretest probability of PE have been compared to those already within the existing pretest probability scores. To compare and quantify the predictive value of pleuritic chest pain, substernal chest pain, dyspnea, estrogen use, family history of PE, and patient history of thrombophilic condition (such as protein C deficiency or Factor V Leiden) to aid in the diagnosis of PE, Courtney et al ⁵ conducted a large prospective study of a heterogenous cohort of nearly 8,000 patients evaluated for PE. Of these patients, 7.2% were found to have a PE.

The group found that the odds ratio (OR) for pleuritic chest pain was 1.53, which is higher than the OR for both hemoptysis and tachycardia. Of note, pleuritic chest pain IS NOT, and hemoptysis and tachycardia ARE variables included in the Wells score.

Don’t forget thrombophilia and family history of PE

Consistent with known decision rules, the variables with the strongest associations with PE were a patient history of venous thromboembolism, unilateral lower extremity swelling, recent surgery, estrogen use, oxygen saturation less than 95%, active cancer, and patient history of thrombophilia. ⁵ Besides pleuritic chest pain, the authors found that the other two variables not included in clinical decision rules with useful ORs were a personal history of non-cancer related thrombophilia (OR 1.99) and a family history of PE (OR 1.51). Failing to take into account the personal and family history may lead to missing the diagnosis of PE.

Using the PERC rule inappropriately: Pregnancy

No clinical prediction rule has been validated for use in pregnant patients. Although the N numbers were small for pregnancy and postpartum status, they concluded that the PERC rule should not be used in isolation to rule out PE in patients who are either pregnant or postpartum.

Using the PERC rule inappropriately: High-prevalence PE populations

It is important to note that the PERC rule was never intended to be applied to anything but a low risk group of patients determined either by clinical gestalt or by the Wells PE score. ⁶ In fact, the meta-analysis ² found some heterogeneity in the PERC rule sensitivity to exclude PE. Two studies from European populations with a prevalence of PE ranging from 21-30% found that a negative PERC rule combined with the low risk Revised Geneva Score only reduced the prevalence of PE in the studied patients to 6%. ^7,8 Only in one of these studies did the PERC rule combined with clinical gestalt reduce the prevalence of PE down to nearly zero.⁸

The prevalence of PE in your community will determine the negative predictive value of the PERC rule where you are practicing. It is suggested that the PERC rule only be utilized where the prevalence of PE is <7%. ⁹ Most of the well-designed PE literature indicates that the PE prevalence in the U.S. is around 6%. ¹

Using the PERC rule inappropriately: Abnormal vital signs

Although intended to be a triage tool, the Wells score doesn’t specify whether you should include an isolated tachycardic pulse rate. One study demonstrated that normalization of vital signs does NOT reduce the probability of a PE in ED patients. So we should use the most abnormal vital signs when risk stratifying patients for PE. ¹⁰ Furthermore, remember that beta blockers may mask tachycardia and can theoretically alter your Wells score or inclusion within the PERC rule. So for the PERC rule, hypoxemia or tachycardia at ANY point during the evaluation is a positive point.

Teaching Points

1. Gestalt or some form of risk stratification should be employed first before using the PERC rule, which is reserved for low pretest probability cases.
2. Because the negative LR of the PERC rule is 0.17, this allows you to have a maximum pretest probability of about 10.7% to apply the PERC rule to risk stratify your patient down to the standard risk of 2% (see below).
3. The maximum suggested prevalence for PE in order to use the PERC rule is 7%.
4. Pleuritic chest pain may increase the likelihood of PE more than some variables within existing decision rules.
5. The PERC rule should not be used in isolation to rule out PE in pregnant or postpartum patients.
6. The PERC rule includes hypoxemia or tachycardia at any point during the evaluation.

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