emergency & critical caredecision rule

PERC Rule (Pulmonary Embolism Rule-out Criteria)

The PERC rule identifies very-low-risk patients in whom pulmonary embolism can be excluded without D-dimer testing or imaging. All 8 criteria must be negative for PE to be ruled out. The rule applies ONLY to patients with LOW clinical pretest probability.

inputs

Positive if ≥50
years
Positive if ≥100
bpm
Positive if <95%
%

✓ when to use

Apply ONLY in patients where PE has been considered but clinical pretest probability is LOW (gestalt or Wells <2). If all 8 criteria are negative, no further workup for PE is required. This avoids unnecessary D-dimer testing and the cascade of false-positive imaging it can trigger.

✗ when not to use

Do NOT apply PERC in patients with moderate or high pretest probability for PE — it was derived and validated exclusively in the low pretest probability population. A common error is applying PERC to 'rule out PE' in patients where clinical suspicion is not already low. PERC is a rule-out tool for the lowest-risk subgroup, not a screening tool for all comers.

clinical pearls

  • PERC only applies when your pretest probability is ALREADY LOW. The most common misuse is applying PERC to patients where PE is genuinely suspected — in those patients, go straight to D-dimer or imaging.
  • Think of PERC as a 'test before the test'. If a patient triggers PE consideration but your clinical gestalt says it's unlikely, PERC can confirm that no testing is needed — saving the patient from a D-dimer that would probably be falsely positive (especially in elderly, post-operative, or pregnant patients).
  • All 8 criteria must be negative. A single positive criterion makes the rule inapplicable. There is no 'partial PERC' or 'PERC score' — it is strictly binary.
  • PERC was developed in the US ED population. Its applicability in other healthcare systems depends on the baseline prevalence of PE in the testing population. In populations with very low PE prevalence, PERC adds less incremental value.
  • The O₂ saturation criterion uses room air values. If the patient is already on supplemental oxygen, you cannot assess this criterion and PERC cannot be applied.

related tools

wells-pePretest probability assessment before PERCage-adjusted-ddimerIf PERC positive, consider age-adjusted D-dimeryears-peAlternative PE diagnostic algorithmspesiPrognostic stratification if PE confirmed