Scores & indices
PERC rule calculator
Eight bedside questions that can end a PE work-up before it starts — but only when your gestalt already says low risk.
How to read the result
PERC criteria falls into these published bands.
A 54-year-old with pleuritic chest pain, normal observations on room air, and a genuinely low-risk story — but aged over 50.
PERC positive — 1 criterion
One criterion is enough: PERC cannot rule out PE here, and the work-up returns to the usual Wells and D-dimer pathway.
Who this is for
For emergency clinicians facing a presentation that has raised the PE question without much conviction — the younger patient with pleuritic chest pain and normal observations. When clinical gestalt puts the probability below ~15% and all eight criteria are absent, PE is ruled out with no D-dimer at all, sidestepping the false-positive cascade to CT angiography.
How it works
What it does
The Pulmonary Embolism Rule-out Criteria: eight bedside findings which, when all absent in a patient whose clinical gestalt is already low (< 15%), put the probability of PE below the threshold where testing helps.
The key discipline
PERC is only valid after you have judged the patient low risk — it cannot be applied to moderate- or high-suspicion presentations, and a single positive criterion simply returns you to the usual Wells/D-dimer pathway.
Why it exists
D-dimer false positives in low-risk patients drive CT angiograms whose harms (radiation, contrast, overdiagnosis) outweigh the tiny miss rate. Reference: Kline et al., 2004.
Built on published evidence
- Kline et al., J Thromb Haemost 2004
Codification last reviewed 2026-08-28, and checked against published reference values in automated tests on every release — see the full evidence ledger.
Questions clinicians ask
Can I apply PERC to a moderate-risk patient?
No. PERC is only valid after you have judged the patient low risk (below ~15% pre-test probability). It is a rule-out for patients you were already close to not testing, not a screening tool for every suspected PE.
What does a positive PERC mean?
Only that the shortcut is closed. A single positive criterion does not diagnose PE — it returns you to the standard pathway: assess probability with the Wells score and proceed to D-dimer or imaging as indicated.
Why not just send a D-dimer anyway?
Because in low-risk patients D-dimer false positives drive CT pulmonary angiograms whose harms — radiation, contrast, overdiagnosis — outweigh the tiny miss rate. That trade-off is exactly why the rule exists (Kline et al., 2004).
What are the eight criteria?
Age ≥ 50, heart rate ≥ 100/min, SaO₂ below 95% on room air, unilateral leg swelling, haemoptysis, surgery or trauma under general anaesthesia in the past four weeks, prior DVT or PE, and oestrogen use. All eight must be absent for a negative PERC.
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