PERC Rule Assessment
⚠️ PREREQUISITE — Low Pre-Test Probability Required
PERC is only valid if pre-test probability of PE is LOW (Wells PE score ≤4, or gestalt clinical impression <15%). Do NOT apply PERC to moderate or high pre-test probability patients — proceed directly to D-dimer or CT-PA.
Tap each criterion that is PRESENT (positive findings). PERC is negative only if ALL 8 are absent.
Positive PERC Criteria
0 / 8
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PERC Rule — Complete PE Workup Algorithm
Suspected PE (dyspnoea, pleuritic pain, tachycardia)
↓
Assess Pre-Test Probability (Wells PE Score or gestalt)
↓
┌─────┴─────┐
LOW MODERATE / HIGH
(Wells ≤4) (Wells >4)
↓ ↓
Apply PERC D-dimer OR
CT Pulmonary Angiogram
↓
All 8 PERC negative?
↓
YES → PE EXCLUDED. No further workup.
NO → Send D-dimer.
D-dimer negative → PE excluded.
D-dimer positive → CT-PA.
Why PERC Matters — Avoiding Over-Testing
Pulmonary embolism is over-investigated in emergency departments globally. Up to 40% of patients who receive CT-PA for PE have a low pre-test probability and negative D-dimer — they could have been PERC-negative and discharged without radiation exposure, contrast risk, or cost. The PERC rule was validated to safely exclude PE in low-risk patients with a miss rate of <2% — equivalent to the accepted threshold for "safe" PE rule-out.
When NOT to Use PERC
- Moderate or high pre-test probability (Wells PE score >4) — go directly to D-dimer or CT-PA
- Patient is haemodynamically unstable — PE likely, urgent CT-PA or echocardiogram
- Pregnancy — D-dimer unreliable, use clinical judgement, ultrasound compression, V/Q scan preferred
- Post-surgery or immobilisation in past 4 weeks — higher baseline risk, PERC not validated
- Known thrombophilia or prior VTE — automatic higher risk category
PERC vs D-dimer vs Wells Score
- Wells PE score: Calculates pre-test probability (low/moderate/high). Should be applied first. Low Wells (≤4) allows PERC application
- PERC rule: Applied AFTER establishing low pre-test probability. Allows exclusion of PE without D-dimer in truly low-risk patients
- D-dimer: Very sensitive (>95%) but non-specific. Elevated in infection, pregnancy, post-op, malignancy, inflammation. Age-adjusted D-dimer: threshold = age × 10 µg/L (for age >50)
- CT Pulmonary Angiogram: Gold standard for PE confirmation. Risk of contrast nephropathy, radiation (5–10 mSv), gadolinium allergy. Reserve for positive D-dimer in appropriate pre-test probability
Related Calculators
⚠ Medical Disclaimer: PERC rule is only valid in patients with a low pre-test probability for PE (Wells ≤4 or gestalt <15%). A negative PERC rule does not completely exclude PE — the miss rate is ~1.4%. Always apply clinical judgement. If there is clinical concern despite a negative PERC, proceed with D-dimer and/or CT-PA.