Stop Before the CT: Are You Using PERC or Wells Correctly?

Clinical Scenario

A 35 year old woman with no past medical history presents to the ED with pleuritic chest pain and shortness of breath.

She is tachypenic but has a normal heart rate and oxygen saturation. She denies leg pain or swelling, or h/o DVT/PE.. She uses oral contraceptives. Her EKG is normal. Cardiac, lung, and extremity exam are unremarkable. You are considering using the Wells' Criteria and/or PERC Rule. Where do you start?

Summary

Use Wells' first if you're unsure about PE likelihood. Use PERC only if you're already thinking "this patient probably doesn't have a PE", or if Wells' determines your patient to be low risk.

Both tools reduce unnecessary imaging when applied correctly.

Tool Comparison

	Wells'	PERC
Purpose	Risk stratification tool to assess the probability of PE.	Rule-out tool to exclude PE in low-risk patients without further testing.
Use When...	…you suspect PE and want to determine pretest probability.	…your patient is already considered low risk for PE (i.e. low clinical suspicion or determined to be low-risk by Wells').
Scoring	Three tier (low, moderate, high) or Two tier (PE unlikely vs. likely).	Not a scoring system; patient must meet all 8 criteria to rule out PE.
Performance	Sensitive for identifying patients needing further work-up. Miss rate is low if used correctly.	High sensitivity, but lower specificity. Best used to rule out, not rule in.
Workup	Three tier: Low risk → d-dimer* Moderate risk → d-dimer* or CT-A High risk → CT-A Two tier:** PE unlikely → d-dimer PE likely → CT-A	If all 8 PERC criteria met, risk of PE is <2% and no further testing needed.
Common Pitfalls	Wells' is not meant to diagnose PE but to guide workup by predicting pre-test probability of PE and appropriate testing to rule out the diagnosis. “PE most likely” is subjective — influenced by clinician experience and patient context. Use with thoughtful clinical judgment.	Even if just one criteria does not apply → can not rule out PE using PERC. This is unidirectional: while PERC negative typically allows the clinician to avoid further testing, failing the rule doesn't force the clinician to order tests. Using PERC in moderate/high-risk patients may lead to false reassurance.
*Ensure you are using high-sensitivity d-dimer if applying it to moderate risk patients **While both two and three tier models are accepted, guidelines appear to favor the two tier model, which utilizes only the high sensitivity d-dimer and more conservative risk stratification; “intermediate” risk patients are thought to be still too high risk to be evaluated without further risk stratification.

Case Resolution

You determine that your patient is determined to be low risk by Wells' Criteria, and proceed to PERC Rule. As she is on OCP’s, PE cannot be excluded using PERC rule. Therefore, a d-dimer is ordered and comes back negative. Her symptoms resolve upon re-evaluation and she is safely discharged!