How it's calculated
If all 8 criteria are absent in a patient the clinician has already judged to be low pre-test probability for PE, the PERC rule allows PE to be ruled out without D-dimer testing or imaging — the rate of missed PE in this scenario is considered low enough (under ~2%) to forgo further testing.
How PERC is applied
| Step | Action |
|---|---|
| 1. Clinical gestalt | Clinician must already judge the patient low pre-test probability for PE |
| 2. Apply PERC | Check all 8 criteria |
| 3a. All 8 negative | PE ruled out — no further testing needed |
| 3b. Any criterion positive | PERC not met — proceed to D-dimer (then imaging if positive) |
Clinical use
- PERC is only valid in patients already assessed as low pre-test probability for PE (e.g. low Wells score) — applying it to moderate or high pre-test probability patients is a misuse of the rule and can miss clinically significant PE.
- Its main value is avoiding unnecessary D-dimer testing (and the downstream imaging cascade from false-positive D-dimers) in genuinely low-risk patients, reducing radiation exposure, contrast use, and cost.
- PERC was derived and validated in emergency department populations — its performance in other settings (inpatient, primary care) is less well established.
- A single positive PERC criterion is enough to fail the rule — it's an all-or-nothing test, not a points-based score like Wells.
Frequently asked questions
What is the PERC rule used for?
It helps decide whether a patient already judged to be low pre-test probability for pulmonary embolism can have PE ruled out clinically, without needing a D-dimer test or imaging, when all 8 criteria are absent.
Can PERC be used on any patient with chest pain or breathlessness?
No — it's only valid after a clinician has already judged the patient to be low pre-test probability for PE (for example via clinical gestalt or a low Wells score). Applying PERC to moderate or high-probability patients is not appropriate and can miss PE.
What happens if even one PERC criterion is positive?
The rule is not met — PE cannot be ruled out on this basis, and the usual next step (D-dimer testing, followed by imaging if positive) should proceed as it would without PERC.
Why use PERC instead of just ordering a D-dimer for everyone?
In genuinely low pre-test probability patients, D-dimer has a high false-positive rate, which drives unnecessary CT scans (with radiation and contrast risk). PERC avoids this cascade entirely in patients where the rate of missed PE without any testing is already very low.
References
- Kline JA, et al. Clinical criteria to prevent unnecessary diagnostic testing in emergency department patients with suspected pulmonary embolism. J Thromb Haemost. 2004;2:1247-1255.
- Kline JA, et al. Prospective multicenter evaluation of the pulmonary embolism rule-out criteria. J Thromb Haemost. 2008;6:772-780.
- Singh B, et al. Diagnostic accuracy of pulmonary embolism rule-out criteria: a systematic review and meta-analysis. Ann Emerg Med. 2012;59:517-520.