PERC Rule Calculator
PERC Rule Calculator
Apply the Pulmonary Embolism Rule-out Criteria to avoid any further testing — not even a D-dimer — in patients whose gestalt pre-test probability of PE is already low.
PERC Rule
8 items → 0-8All eight criteria absent
Scoring
- criteria
- age ≥50, heart rate ≥100, SpO₂ <95% on room air, haemoptysis, exogenous oestrogen use, previous DVT or PE, unilateral leg swelling, recent surgery or trauma requiring hospitalisation
- 0 of 8
- PERC negative
- 1 or more of 8
- PERC positive — the rule cannot be applied to exclude PE
Worked example
All eight criteria absent
Age under 50, heart rate under 100, saturation 95% or above, no haemoptysis, no oestrogen use, no previous DVT/PE, no leg swelling, no recent surgery or trauma
0 criteria present → PERC negative
PE risk is below about 2% — no further testing needed
PERC criteria
| Criterion | Fails PERC if |
|---|---|
| Age | 50 or over |
| Heart rate | 100 or more |
| Oxygen saturation | Below 95% on room air |
| Haemoptysis | Present |
| Exogenous oestrogen | Currently taken |
| Previous DVT or PE | Present |
| Unilateral leg swelling | Present |
| Surgery or trauma requiring hospitalisation | Within the last 4 weeks |
A rule-out tool with one strict precondition
PERC is only valid when applied first to a filter it does not itself provide: the clinician’s own gestalt pre-test probability of pulmonary embolism must already be low, conventionally under about 15%. It was derived and validated exclusively in that population, and its performance has not been established — and should not be assumed — outside it.
Within that population, if all eight criteria are absent, the probability of PE falls below roughly 2%, low enough that the rule’s authors judged no further testing was warranted on this indication — not imaging, and not even a D-dimer. If any single criterion is present, PERC is exhausted; it offers no partial information and does not adjust a probability the way a weighted score does. Testing then proceeds by whatever pathway the clinical probability otherwise indicates.
The commonest error is sequencing: applying PERC as a screening test to every patient with chest pain or breathlessness, rather than reserving it for the subset already judged low-risk by clinical gestalt. Applied to a patient with a moderate or high pre-test probability, a PERC-negative result is meaningless and using it to stop the work-up in that group is unsafe — the rule was never validated to perform that function, and doing so has been shown to miss clinically important pulmonary emboli.
Frequently asked questions
What does PERC negative mean?
All eight PERC criteria are absent, which — in a patient already judged low pre-test probability — puts the risk of PE below about 2%. No further testing, including D-dimer, is needed on this indication.
Who can PERC be applied to?
Only patients whose clinical gestalt pre-test probability of PE is already low, conventionally under about 15%. It was derived and validated exclusively in that group.
What happens if one PERC criterion is present?
The rule cannot exclude PE. A single positive criterion makes the patient PERC positive, and testing proceeds via D-dimer or imaging as the clinical pathway otherwise directs.
Is it safe to use PERC on every patient with chest pain?
No. Applying PERC to a moderate- or high-probability patient is a sequencing error the rule was never validated for, and has been shown to miss clinically important pulmonary emboli.
Related calculators
References
- Kline JA et al. Clinical criteria to prevent unnecessary diagnostic testing in emergency department patients with suspected pulmonary embolism. J Thromb Haemost. 2004;2(8):1247–55.
- Kline JA et al. Prospective multicenter evaluation of the pulmonary embolism rule-out criteria. J Thromb Haemost. 2008;6(5):772–80.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.