Tiny Tips: PERC Rule

In Medical Concepts, Tiny Tips by Brent Thoma7 Comments

The “Tiny Tips” series aims to provide helpful memory aids for remembering difficult to recall exam material for emergency medicine. You can get all of them compiled in an easy-to-review way by downloading the Tiny Tips deck of Boring Cards. This post outlines a mnemonic to help remember the PERC rule (Pulmonary Embolism Rule-out Criteria).

PERC is a useful clinical decision rule to help rule-out pulmonary embolism in patients a clinician’s gestalt suggests are low risk. It was developed by Kline in 2004 and validated by him in a prospective, multicenter trial in 2008. As outlined by very well by Chris Nickson on Life in the Fast Lane in much greater depth, a very low risk patient (estimated at <15%) that is PERC(-) is as likely to be harmed by the work-up (think cancer, contrast nephropathy, anaphylaxis) as they are to have a PE in the first place (they are below the test threshold). Knowing this rule is helpful both clinically and for exam purposes (although I’d highly recommend using a smartphone checklist when applying the rule in clinical practice). I remember it using the (aptly named) HAD CLOTS mnemonic that I have added to the Boring Cards deck.

  • H – Hormone (estrogen) use
  • A – Age > 50
  • D – DVT or PE history (have they HAD CLOTS?)
  • C – Coughing blood
  • L – Leg swelling disparity
  • O – O2 sats < 95%
  • T – Tachycardia (>100bpm)
  • S – Surgery or Trauma (recent)

The subtleties of this clinical decision tool are discussed well elsewhere (also see the unfortunately unFOAMy but still awesome 2013 update on EM:RAP), but it is important to apply it with the gestalt criteria for use (it is bolded, italicized and colored above) and remember that it is only is an all-or-none proposition used to RULE OUT PE (it’s not for risk-stratification!). While a low-scoring patient may be a low risk patient, if any of the criteria are positive they are not PERC(-) and an appropriate combination of Wells/Geneva/DDimer/CTPA should be used to rule out this diagnosis.

This mnemonic may not be new to many of you, but it is one of the ones I use most frequently and I wanted to get it into the deck so I would quit forgetting what the damn C stands for!

Brent Thoma

Editor at CanadiEM
Dr. Brent Thoma is a medical educator, blogging geek, and trauma/emergency physician who works at the University of Saskatchewan College of Medicine. He founded BoringEM and is the CEO of CanadiEM.