Emergency Medicine
PERC Rule
Rules out PE in low-risk patients when all criteria are absent.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The Pulmonary Embolism Rule-out Criteria (PERC) is a set of eight clinical criteria used to exclude PE in patients who are already judged to be at low pre-test probability. The eight items are age ≥50, heart rate ≥100, oxygen saturation below 95% on room air, unilateral leg swelling, haemoptysis, recent surgery or trauma, prior PE or DVT, and oestrogen (oral contraceptive or hormone replacement) use. If all eight are absent, the probability of PE is low enough that no further testing — including D-dimer — is needed. It was designed to reduce unnecessary investigation in low-risk patients.
Interpreting the result
A PERC-negative result means all eight criteria are absent and, in a patient already low risk by clinical gestalt, PE can be excluded without a D-dimer. A PERC-positive result (any one criterion present) does not diagnose PE but means PERC cannot exclude it, so the standard pathway (typically D-dimer, then imaging if needed) continues. PERC is therefore a rule-out tool used only at the low-probability end of the spectrum.
Worked example
A 35-year-old with pleuritic chest pain, normal observations, no leg swelling, no haemoptysis, no recent surgery, no prior clot and not on hormones is PERC-negative. Because the clinician already judged them low risk, PE can be excluded without further testing.
Critical actions
PERC only applies when clinical gestalt is already low risk. It does not apply to moderate/high pre-test probability.
Pearls / pitfalls
- PERC only applies when clinical gestalt is already low risk — it must not be used in moderate or high pre-test probability patients.
- A single positive criterion means PERC cannot exclude PE; it does not mean the patient has a PE.
- It is a rule-out, not a rule-in, tool — being PERC-positive simply returns the patient to the standard diagnostic pathway.
- Pregnancy and oestrogen use are important considerations; hormone use is itself one of the criteria.
Evidence & validation
Derived and validated by Kline et al., PERC has been confirmed in subsequent studies (including the PROPER trial) and is incorporated into emergency-medicine practice as a safe way to avoid unnecessary D-dimer testing in low-risk patients.
Frequently asked questions
When can PERC be used?
Only in patients already judged low risk for PE by clinical gestalt. It is not valid in those with moderate or high pre-test probability, where the standard pathway applies.
What does PERC-negative mean?
It means all eight criteria are absent. In a patient already low risk, this allows PE to be excluded without a D-dimer or imaging.
Does a positive PERC mean the patient has a PE?
No. A positive result simply means PERC cannot exclude PE, so the usual diagnostic pathway (often D-dimer then imaging) continues. It is not a diagnosis.
How does PERC fit with the Wells score?
The Wells score estimates pre-test probability; PERC is applied to those already judged low risk to safely avoid a D-dimer. They are complementary steps in the PE pathway.
Why is hormone use a criterion?
Oestrogen-containing contraception and hormone replacement raise venous thromboembolism risk, so their use is one of the eight criteria that, if present, prevents PE being ruled out by PERC.
References
- Kline JA, Mitchell AM, Kabrhel C, et al. Clinical criteria to prevent unnecessary diagnostic testing in emergency department patients with suspected pulmonary embolism. J Thromb Haemost. 2004;2(8):1247–1255.
- Freund Y, et al. Effect of the Pulmonary Embolism Rule-Out Criteria on subsequent thromboembolic events among low-risk emergency department patients (PROPER). JAMA. 2018;319(6):559–566.
About the creator
Kline JA et al.
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