Overheard a conversation about PERC and Wells Criteria and common misconceptions, so here is a review, whether you want it or not:
In short, PERC is a score used to help rule out PE in low pretest probability patients, without performing testing. This is ONLY to be used in patients that you feel have a LOW likelihood of PE. Specifically, gestalt pretest probability < 15%. In this population, if all 8 criteria are negative, no testing is necessary (PE likelihood < 2%). Not sure how you give a number to your gestalt, so the take home here is you apply PERC to patients that you have to include PE in the differential, but feel that there is no way that the symptoms are caused by PE. PERC SUPPORTS YOUR DECISION NOT TO TEST. Exclusion criteria are:
Known thrombophilia
Strong family history of thrombosis
Concurrent beta-blocker use (could blunt reflex tachycardia)
Transient tachycardia
Patients with amputations
Massively obese patients in whom unilateral leg swelling could not be assessed
Patients with baseline SaO2 of < 95%
Wells Criteria Objectifies the risk of PE. It is not based on gestalt, but rather a set of criteria that helps you to determine pretest probability of PE. Wells not only gives a numerical score to quantify risk, but that score also guides your further workup. The goal of the Wells Criteria is to objectively stratify risk to determine who a d-dimer is sufficient to rule out PE, and who needs a CTA.
Dr. Wells on the use of his scores:
The model should be applied only after a history and physical suggests that venous thromboembolism is a diagnostic possibility. it should not be applied to all patients with chest pain or dyspnea or to all patients with leg pain or swelling. This is the most common mistake made. Also, never never do the D-dimer first [before history and physical exam]. The monster in the box is that the D-dimer is done first and is positive (as it is for many patients with non-VTE conditions) and then the physician assumes that VTE is now possible and then the model is done. Do the history and physical exam first and decide if VTE is a diagnostic possibility!
When applying the Wells Criteria, there are two models available. The two tier model is preferred. The Christopher study found a 12.1% incidence of PE in the “unlikely” group and a 37.1% incidence in the “likely” group. Applying their two-tier decision rule resulted in a PE miss rate of only 0.5% on 3 month follow up.
Three Tier Model
1. Patient is determined to be low risk (<2 points:1.3% incidence PE): consider d-dimer testing to rule out Pulmonary embolism. Alternatively consider a rule-out criteria such as PERC.
If the dimer is negative consider stopping workup.
If the dimer is positive consider CTA.
2. Patient is determined to be moderate risk (score 2-6 points, 16.2% incidence of PE): consider high sensitivity d-dimer testing or CTA.
If the dimer is negative consider stopping workup.
If the dimer is positive consider CTA.
3. Patient is determined to be high risk (score >6 points: 37.5% incidence of PE): consider CTA. D-dimer testing is not recommended.
Two Tier Model (PREFERRED)
1. Patient risk is determined to be “PE Unlikely” (0-4 points, 12.1% incidence of PE): consider high sensitivity d-dimer testing.
If the dimer is negative consider stopping workup.
If the dimer is positive consider CTA.
2. Patient risk is determined to be “PE Likely” (>4 points, 37.1% incidence of PE): consider CTA testing.
Note: the original three tier model includes the option of using PERC for low risk patients instead of the d-dimer. The newer, preferred two tier model uses high sensitivity d-dimer for lower risk group and CTA for higher risk group.
