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
- Age < 50 years
- HR < 100 bpm
- Room air oxygen saturation > 94%
- No prior history of DVT or PE
- No recent trauma or surgery
- No hemoptysis
- No exogenous estrogen
- 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:
- Pleuritic chest pain (56% of 114 PERC negative patients vs 38% of 1,766 PERC positive patients)
- Pregnancy (4% of 114 PERC negative patients vs 1% of 1,766 PERC positive patients)
- 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.
- Gestalt or some form of risk stratification should be employed first before using the PERC rule, which is reserved for low pretest probability cases.
- 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).
- The maximum suggested prevalence for PE in order to use the PERC rule is 7%.
- Pleuritic chest pain may increase the likelihood of PE more than some variables within existing decision rules.
- The PERC rule should not be used in isolation to rule out PE in pregnant or postpartum patients.
- The PERC rule includes hypoxemia or tachycardia at any point during the evaluation.
Expert Peer Review
Q: What do you consider to be the limitations of the PERC rule?
Dr. Kline: Physicians have to use it correctly (meaning look at the vital signs and ask all the questions). Do not try to memorize the components. Look them up-- memorizing decision rules, PERC, NEXUS, PECARN, or any of them is not necessary. This is not like a breath holding contest or a hotdog eating contest. And PERC is not validated in pregnant patients.
Q: Is there any more data on the PERC negative patients who will actually end up getting a diagnosis of PE?
Dr. Kline: Not beyond the letter I published in Annals last year: Kline JA, et al. Clinical features of patients with pulmonary embolism and a negative PERC rule result. Ann Emerg Med. 2013 Jan;61(1):122-4. Pubmed
Q: What about using a modified PERC for pregnant or post-partum patients?
Dr. Kline: Trying to validate in pregnancy, and working on it.
Q: Are there any future directions you are taking the PERC rule, such as modifying it for certain populations? Or do you feel that the PERC rule has been validated enough (excluding pregnant and post-partum patients)?
Dr. Kline: We are trying to validate the PERC rule in pregnancy. The PERC rule has been validated enough. See the meta-analysis by Singh et al (Pubmed). Here are excerpts from that study:
\"We found that when the pretest probability is low, PERC are highly sensitive in predicting pulmonary embolism, and D-dimer testing is thus unnecessary. These findings are at a confidence of what is considered “level 2 evidence,” ie, demonstrated accuracy in either 1 large prospective study including a broad spectrum of patients and clinicians or validated in several smaller settings that differed from one another.\"
\"Our meta-analysis reports consistent high sensitivity and negative predictive value of PERC, with missed pulmonary embolism in just 0.5% of patients. Two of the included studies (Hugli et al and Righini et al) report a higher frequency of missed pulmonary embolism and have raised concern about the reliability of PERC. However, their higher failure rate likely results from the higher pulmonary embolism prevalence observed in their European settings. The threshold for pulmonary embolism diagnostic imaging in the United States is substantially lower than that in Europe, presumably because of the higher litigation risk. The PERC rule was developed for use in low-probability settings.\"