What the PERC rule is for
Pulmonary embolism (PE) is feared because it can be fatal, so emergency physicians test for it often. But D-dimer tests are frequently falsely positive, leading to CT scans with radiation, contrast and incidental findings. The Pulmonary Embolism Rule-out Criteria (PERC), published by Jeffrey Kline in 2004, identify low-risk patients who need no testing at all.
The eight criteria
PE can be ruled out without D-dimer if all of the following are true in a patient already judged to have a low clinical probability:
- Age under 50
- Heart rate under 100 beats per minute
- Oxygen saturation of 95% or more on room air
- No hemoptysis (coughing up blood)
- No estrogen use (oral contraceptives, hormone therapy)
- No prior DVT or PE
- No unilateral leg swelling
- No surgery or trauma requiring hospitalization in the past 4 weeks
Worked example
A 35-year-old with pleuritic chest pain after a cold, heart rate 88, SpO₂ 98%, not on the pill, no history of clots, no leg swelling and no recent surgery: 0 criteria present. If the clinician’s overall impression is low probability, PE is ruled out without further testing.
When not to use it
- When the clinical probability is not low: use the Wells score and D-dimer or imaging instead.
- In pregnancy and the postpartum period, where specific algorithms apply.
- In inpatients, who were not part of the validation studies.
How safe is it?
In US emergency departments, the rate of missed PE in PERC-negative, low-probability patients was below 1% in validation studies. The French PROPER trial (2018) randomized patients across 14 emergency departments and confirmed that the PERC strategy was not inferior to usual care while reducing CT scans.
Where PERC fits
- Clinical gestalt or a formal score suggests low probability.
- PERC negative: stop, PE excluded.
- PERC positive: measure D-dimer, with the age-adjusted cutoff after 50.
- D-dimer positive: CT pulmonary angiography.
Limits
PERC relies on the clinician’s initial judgment of low probability. A patient who looks unwell, has unexplained low oxygen or has risk factors not listed, such as active cancer, should not be “PERC-ed out”.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.