PERC negative?

by Bob Stuntz

How appropriate with the recent podcast…

The June 2012 issue of Annals of Emergency Medicine is actually full of interesting articles.  With the recent podcast giving you an introduction to clinical decision rules, my attention was drawn to an article entitled “Diagnostic Accuracy of Pulmonary Embolism Rule-Out Criteria: A Systematic Review and Meta-analysis.”  

An introduction for those not familiar: The Pulmonary embolism Rule out criteria (PERC) (1) were developed to decide who amongst low risk patients could have any testing omitted in ruling out PE.  The criteria are as follows: 

Age < 50

HR < 100 (any one documented rate above 100 in their visit counts)

Pox > 94%

No prior history of DVT/PE

No hemoptysis

No trauma or surgery

No exogenous estrogen

No clinical signs suggesting DVT/PE

If you meet all those criteria, the risk of PE in a population you have deemed low risk is minimal, and further workup may be avoided.  In this article, they looked at 12 studies and almost 14,000 patients.  The short of it is that PERC was found to be very sensitive in the pooled analysis.  Sensitivity was about 97%, and specificity of 23%.  So again, a negative result means very low likelihood of disease. However, this comes at a cost of specificity, and the low specificity indicates patients without DVT/PE will often have false positives.  

So I like PERC with one MAJOR caveat:  people do not apply this rule correctly.  This is another one of my big beefs with clinical decision tools.  In order to truly use any of these rules, you have to know how to apply it and when it is acceptable to use.  Remember, PERC was developed to help avoid the need for workup in LOW RISK patients, or patients who you deem to have a very low pretest probability of disease.  So you can be PERC negative but still need a workup.  And remember that PERC is different from Wells, in that Well s helps you decide HOW to work someone up, not if you can avoid workup.  

So lets say you have a 45 year old asthmatic who had sudden onset SOB earlier.  She thinks it is her asthma.  VS are normal, respiratory rate is normal, and Pox is 96% on RA.  Her HR is 90.  She has clear lungs with good air movement.  She is PERC negative.  What do you do?

She does not meet PERC, but you have someone with SOB and clear lungs with a borderline sat and Pox in whom the cause of SOB does not clinically fit with asthma  .  You need to work her up.  

So in summary: Low risk patients who you deem have a low pretest probability may rule out workup for DVT/PE if PERC is all negative.  However, if your pretest probability is high, you need to do a workup even if PERC is negative.  Meeting any PERC criteria means you need a workup for DVT/PE.  

Again, know how to use these rules and what they mean before you go throwing them around.  

1.  Kline, JA; Mitchell, AM; Kabrhel, C; Richman, PB; Courtney, DM (2004). “Clinical criteria to prevent unnecessary diagnostic testing in emergency department patients with suspected pulmonary embolism”Journal of Thrombosis and Haemostasis 2 (8): 1247–1255