Why We Use the PERC Rule

On the Shoulders of Giants

Perks of PERC
This was the magnum opus of Dr. Kline, an enormous prospective study of over 8000 patients presenting to the ED with clinical suspicion of PE.  The goal was to determine if clinical criteria could rule out the disease, meaning <2% chance of venous thromboembolism (VTE) or death in patients who had negative PE rule-out criteria (PERC).  There are 8 clinical variables.  Plus it must only be used in a patient in whom your clinical gestalt for PE is low, defined as <15% pretest probability.  So PERC is: Low gestalt + 8 variables.  Here they are, using the mnemonic HAD CLOTS:

  • Hormones - Is the patient taking exogenous estrogen?
  • Age - Is the patient >/= 50 years old?
  • DVT/PE - Does the patient have prior history of DVT/PE?
  • Coughing blood - Does the patient have hemoptysis?
  • Lower extremity - Does the patient have unilateral lower extremity edema?
  • O2 sat - Does the patient have SpO2 < 95% on room air?
  • Tachycardia - Does the patient have heart rate >/= 100 bpm?
  • Surgery/trauma - Does the patient have history of surgery or trauma requiring hospitalization in the past 4 weeks?

If yes to any of these questions, the patient is not PERC negative.  That doesn't mean you are compelled to do a CT, but it does mean they are not ultra-low risk and can't be ruled out with clinical criteria alone.  If the pretest probability of disease is "PE Unlikely" or "low to intermediate" based on simplified or original Wells Criteria, then a D-Dimer would be appropriate as the next step.

If you can't remember this mnemonic, just use MDCalc.

PERC negative patients in this study had 1% who had VTE or died in the 45-day follow up period (15 had VTE and 1 died = 16/1666).  Of 8138 patients, 20% were classified as PERC negative.  The big deal with this study is that using our clinical judgment and a few objective findings from the history and physical exam, we can rule out PE with no labs and no imaging.  This simplifies the PE workup and reduces radiation exposure.  My one caveat with PERC is to get an ECG on all these people and allow it to color your gestalt.  Deep T-wave inversions in V1-V4, an RSR' pattern in V1, RV strain pattern, S1Q3T3, etc. should alert you that this is not a low gestalt patient and PERC should not be used.

Spoon Feed
The PERC rule is a powerful diagnostic tool.  If you determine a patient has low clinical gestalt for PE and all 8 PERC criteria are negative, then PE has been ruled out without checking a D-dimer or CT.

Another Spoonful

Prospective multicenter evaluation of the pulmonary embolism rule-out criteria.  J Thromb Haemost. 2008 May;6(5):772-80. doi: 10.1111/j.1538-7836.2008.02944.x. Epub 2008 Mar 3.