PERC Rule for Pulmonary Embolism
The Pulmonary Embolism Rule-Out Criteria (PERC) allow clinicians to safely exclude PE in low-risk patients without D-dimer testing or imaging. All 8 criteria must be negative. Only applicable when the clinician’s pre-test probability is already low (<15%).
Apply the PERC Rule
Answer each of the 8 PERC criteria below. All 8 must be “No” to rule out PE without further testing. If any single criterion is “Yes,” PE cannot be excluded by PERC alone — proceed to D-dimer testing. This tool should only be applied when your clinical gestalt already places PE as a low-probability diagnosis (<15%, roughly corresponding to a Wells PE score of ≤1).
PERC is only valid when the clinician’s pre-test probability is already low (<15%). It is not a screening tool and should never be applied to moderate- or high-risk patients. Applying PERC when the pre-test probability is moderate (15–40%) results in an unacceptable miss rate of 5–8%. Always determine whether the patient meets the low-risk prerequisite — typically a Wells PE score of 0–1 — before applying PERC.
Understanding the PERC Rule
The PERC rule was developed by Jeffrey Kline and colleagues in 2004 to address a specific clinical problem: among patients in the emergency department with symptoms that prompt consideration of PE but who are judged to be at low risk by clinical gestalt, is there a way to safely avoid D-dimer testing altogether? D-dimer is highly sensitive but poorly specific, and positive results in low-risk patients lead to unnecessary CTPA scans, radiation exposure, contrast risks, incidental findings, and downstream investigations.
The rule was derived from a dataset of over 21,000 emergency department patients and prospectively validated in a multicentre study of 8,138 patients in 2008. The key finding: among patients with a clinician-estimated pre-test probability below 15% who met all 8 PERC criteria, the rate of PE during 45-day follow-up was 1.0% — below the accepted 2% testing threshold for PE.
The 8 PERC Criteria
All must be “No” to rule out PE:
1. Age ≥50 years
2. Heart rate ≥100 bpm
3. SpO₂ <95% on room air
4. Unilateral leg swelling
5. Haemoptysis
6. Recent surgery or trauma (≤4 weeks)
7. Prior PE or DVT
8. Oestrogen use (OCP, HRT)
The Logic — Binary Pass/Fail
Unlike the Wells score (which is additive), PERC is an all-or-nothing gate. A single positive criterion prevents rule-out. There is no partial score or risk stratification — PERC either fully excludes PE or it doesn’t. This design prioritises sensitivity (not missing PE) over specificity.
Worked example: A 34-year-old woman presents with pleuritic chest pain. HR 88, SpO₂ 98%, no leg swelling, no haemoptysis, no surgery, no VTE history, no oestrogen. Clinician gestalt: PE <15%. All 8 PERC = No → PERC negative → PE excluded.
Key concept — The testing threshold: In emergency medicine, a disease can be safely excluded without testing when the post-test probability would remain below 1.8–2.0% even without the test (the “testing threshold”). PERC was designed to demonstrate that low-risk patients meeting all 8 criteria already have a PE prevalence below this threshold, making D-dimer testing unnecessary — and potentially harmful due to downstream false-positive cascades.
Interpreting the PERC Result
The PERC rule produces a binary result: the patient either passes (all 8 criteria negative) or fails (one or more criteria positive). There is no intermediate category or partial score.
| PERC Result | Positive Criteria | Missed PE Rate | Action |
|---|---|---|---|
| PERC Negative (Pass) | 0 out of 8 | <2% | PE safely excluded — no D-dimer, no imaging needed |
| PERC Positive (Fail) | ≥1 out of 8 | Not excludable | Proceed to D-dimer testing (if Wells ≤4) or imaging (if Wells >4) |
Where PERC Fits in the Diagnostic Pathway
PERC was designed to prevent unnecessary D-dimer testing in low-risk patients. The clinical problem it addresses is the false-positive D-dimer cascade: a low-risk patient gets a D-dimer “just to be safe,” the result is mildly positive (common in elderly, post-surgical, or pregnant patients), and the patient undergoes CTPA that was never indicated, exposing them to radiation, contrast, and incidental findings. PERC breaks this cascade at its origin.
The PERC Criteria in Clinical Context
Each PERC criterion was selected because it independently identifies a feature associated with increased PE probability. Understanding the rationale behind each criterion helps clinicians apply the rule accurately and recognise edge cases.
The incidence of PE increases markedly with age. Below age 50, PE is relatively uncommon (approximately 0.5–1 per 1,000 person-years), while after 50 the incidence rises to 2–7 per 1,000 person-years by age 80. Age ≥50 was the most discriminating single threshold in the PERC derivation cohort and serves as a proxy for the cumulative vascular risk factors, immobility, comorbidity, and prothrombotic changes that accompany ageing.
This threshold means that PERC can only be applied to patients under 50. In older patients with low clinical suspicion, D-dimer (ideally age-adjusted) remains the appropriate next step after clinical risk assessment.
Tachycardia is present in approximately 24–44% of patients with confirmed PE and reflects the haemodynamic response to acute right ventricular pressure overload and increased dead space ventilation. A heart rate ≥100 bpm at initial assessment fails the PERC criterion and also scores +1.5 on the Wells PE model. Importantly, the threshold is ≥100 bpm — a heart rate of exactly 100 bpm fails PERC.
Pain, anxiety, fever, and dehydration can all cause tachycardia in the absence of PE. However, the PERC rule is deliberately conservative — any single positive criterion prevents rule-out, even if the tachycardia has an obvious non-PE explanation. This high sensitivity is by design.
PE causes hypoxaemia through ventilation-perfusion mismatch, increased dead space, and occasionally right-to-left shunting through a patent foramen ovale. An SpO₂ below 95% on room air (not on supplemental oxygen) is associated with increased PE probability and fails the PERC criterion. Notably, approximately 20% of confirmed PE patients have a normal SpO₂ — so a normal SpO₂ alone does not exclude PE.
The measurement must be on room air. If the patient is already receiving supplemental oxygen, the SpO₂ may be falsely normal and the PERC criterion cannot be reliably assessed. In this situation, PERC should be considered failed for this criterion unless supplemental oxygen can be safely discontinued and a room-air reading obtained.
Unilateral leg swelling suggests concurrent deep vein thrombosis, which is the source of embolism in the majority of PE cases. Approximately 70% of patients with PE have evidence of DVT on imaging, though only 25–50% have clinically apparent leg signs. Any asymmetric leg swelling — even if mild — fails the PERC criterion and should prompt consideration of both DVT and PE evaluation.
This criterion assesses clinical signs at the bedside, not imaging results. It does not require formal measurement or comparison with the contralateral limb beyond clinical inspection. Bilateral symmetrical oedema (e.g., from heart failure or nephrotic syndrome) does not fail this criterion.
Haemoptysis: Although uncommon in PE (present in <10% of cases), haemoptysis in the context of dyspnoea or pleuritic chest pain is a specific marker of pulmonary infarction and significantly raises the probability of PE. Any amount of haemoptysis — even blood-streaked sputum — fails the criterion.
Recent surgery or trauma (≤4 weeks): Post-operative and post-trauma immobility, venous stasis, and tissue factor release create a highly prothrombotic state. The 4-week window encompasses the highest-risk period following surgery. This aligns with the Wells PE criterion for immobilisation/surgery (+1.5 points).
Prior PE or DVT: A history of VTE places the patient in a higher baseline risk category for recurrent thromboembolism. Recurrence rates are approximately 5–10% per year after a first unprovoked VTE. Any objectively confirmed prior PE or DVT fails the criterion.
Oestrogen use: Exogenous oestrogen — combined oral contraceptive pills (not progesterone-only), hormone replacement therapy, or other oestrogen formulations — increases VTE risk 2–6 fold through upregulation of coagulation factors and downregulation of natural anticoagulants. This criterion applies to current oestrogen use at the time of presentation.
Special Populations & Limitations
PERC was derived and validated in US emergency departments among ambulatory patients with symptoms prompting consideration of PE. Several important populations are either excluded from its applicability or require particular caution.
Subsegmental PE and PERC: The “missed PE” rate of <2% in PERC-negative patients may include a small proportion of subsegmental PEs that were never diagnosed. However, the clinical significance of many subsegmental PEs is debated, and the accepted testing threshold of 1.8–2.0% already accounts for this possibility. PERC does not claim to exclude all PE — it claims that among PERC-negative low-risk patients, the adverse event rate is below the threshold that would justify testing.
Where PERC Fits: The Full PE Diagnostic Pathway
The following step-by-step pathway shows how PERC integrates with the Wells score, D-dimer testing, and imaging in the evaluation of suspected PE. PERC applies only at one specific point in this pathway.
If the patient has sustained systolic BP <90 mmHg, obstructive shock, or cardiac arrest with suspected PE, this is a high-risk (massive) PE pathway. PERC, Wells, and D-dimer are all irrelevant — proceed to emergency CTPA or bedside echocardiography. Consider empiric systemic thrombolysis.
If haemodynamically stable, proceed to Step 2.
Calculate the Wells score for PE. This determines which pathway to follow:
- Wells 0–1 (Low, 3-tier): Pre-test probability ~1.3%. → Proceed to Step 3 (PERC).
- Wells 2–4 (Moderate, still “PE Unlikely” on 2-tier): Pre-test probability ~8–16%. → Skip PERC. Proceed to D-dimer.
- Wells >4 (“PE Likely”): Pre-test probability ~34%. → Skip PERC and D-dimer. Proceed directly to CTPA.
For patients with Wells 0–1 and a clinical gestalt <15%, apply the 8 PERC criteria:
- All 8 “No” (PERC Negative): PE is safely excluded. No D-dimer, no imaging. Document the clinical reasoning and PERC result. Explore alternative diagnoses for the patient’s symptoms.
- Any criterion “Yes” (PERC Positive): PE cannot be excluded by PERC alone. Proceed to high-sensitivity D-dimer testing. Use age-adjusted thresholds (age × 10 µg/L) for patients over 50.
D-dimer testing is indicated when PERC is positive (fails) or the pre-test probability is moderate (Wells 2–4). A negative high-sensitivity D-dimer safely excludes PE (3-month VTE rate <1%). A positive D-dimer requires imaging — proceed to CTPA or V/Q scan.
Always use the appropriate threshold: standard 500 µg/L for patients ≤50, or age-adjusted (age × 10 µg/L) for patients >50. The age-adjusted approach has been validated in the ADJUST-PE trial and reduces unnecessary imaging by 12–30% in older patients.
CTPA is the first-line imaging modality for PE in most clinical settings: sensitivity ~95–98%, specificity ~97%, with the added benefit of identifying alternative diagnoses. V/Q scintigraphy is preferred when CTPA is contraindicated (contrast allergy, severe renal impairment, pregnancy in some centres).
If PE is confirmed, risk-stratify (haemodynamic status, RV function, troponin, BNP) and initiate anticoagulation. If PE is excluded and an alternative diagnosis is identified on imaging, treat accordingly.
Common Pitfalls & Misapplications
The PERC rule is conceptually simple but frequently misunderstood and misapplied. The following pitfalls represent the most common sources of error.
This is the most dangerous misapplication of PERC. The rule was validated exclusively for patients with a low pre-test probability (<15% clinician gestalt, roughly Wells 0–1). In patients with moderate pre-test probability (Wells 2–6), the PE prevalence is approximately 16% — and the PERC criteria are not sufficiently sensitive to reduce this to below the 2% testing threshold. Using PERC in moderate-risk patients produces a miss rate of 5–8%, which is clinically unacceptable.
Example of the error: A 55-year-old woman with Wells 4 (moderate risk) has no haemoptysis, no surgery, no prior VTE, no oestrogen use. She fails PERC on age alone, but even if she were younger, PERC should never have been applied at this pre-test probability. The correct next step at Wells 4 is D-dimer — not PERC.
PERC is not a screening tool for PE. It should only be applied when the clinician has already considered PE as a diagnostic possibility and judged it to be low-probability. Applying PERC to patients in whom PE was never part of the differential — or conversely, to all patients with chest pain or dyspnoea regardless of clinical suspicion — misuses the rule and can give false reassurance.
The correct sequence is: (1) clinical assessment raises the question of PE, (2) clinician estimates low pre-test probability, (3) PERC is applied as a decision support tool, (4) if all 8 criteria are met, the clinician’s low gestalt is confirmed and no testing is needed. PERC confirms a low-risk impression — it does not create one.
A PERC-negative result applies to the clinical picture at the time of assessment. If the patient’s symptoms worsen, new features develop (e.g., new leg swelling, worsening dyspnoea, haemodynamic deterioration), or new risk factor information comes to light, the pre-test probability may change and PERC may no longer be applicable. Patients should receive clear return precautions and instructions to seek re-evaluation if symptoms change.
Additionally, PERC-negative status does not replace a thorough diagnostic evaluation for alternative causes of the patient’s symptoms. The clinician should still identify and address the actual diagnosis (musculoskeletal pain, anxiety, viral pleuritis, etc.) rather than simply discharging the patient with “not PE.”
The SpO₂ criterion specifies oxygen saturation on room air — not on supplemental oxygen. A patient receiving nasal cannula oxygen with an SpO₂ of 97% may have a true room-air saturation of 88%. If the patient is already receiving supplemental oxygen and cannot safely have it discontinued, the SpO₂ criterion cannot be reliably assessed, and PERC should be considered as failed for this criterion.
Similarly, patients with chronic lung disease (COPD, pulmonary fibrosis) who have a baseline SpO₂ below 95% will automatically fail PERC on this criterion. This is appropriate — these patients have higher PE risk and more complex differential diagnoses that warrant formal evaluation rather than clinical rule-out.
PERC is only valid when the pre-test probability is low (<15%). This prerequisite is a clinical judgement, and it must be documented. Simply documenting “PERC negative” without documenting the pre-test probability assessment is medicolegally and clinically insufficient — it leaves no evidence that the rule was applied to the correct patient population.
Best practice documentation: “PE considered in the differential for this patient with pleuritic chest pain. Clinical gestalt estimates low pre-test probability (<15%). Wells PE score = 0. PERC rule applied: all 8 criteria negative. PE excluded by PERC without further testing. Alternative diagnosis: [diagnosis]. Return precautions given.”
Quick Reference Summary
| Criterion | Threshold | Must Be |
|---|---|---|
| Age | <50 years | No (under 50) |
| Heart rate | <100 bpm | No (under 100) |
| SpO₂ | ≥95% on room air | No (95% or above) |
| Unilateral leg swelling | Absent | No |
| Haemoptysis | Absent | No |
| Recent surgery/trauma | None within 4 weeks | No |
| Prior PE or DVT | No history | No |
| Oestrogen use | Not currently using | No |
The Golden Rule: PERC is a confirmation tool for low clinical suspicion — not a screening tool. Three conditions must be met: (1) PE was considered in the differential, (2) clinician gestalt is <15% (roughly Wells 0–1), and (3) all 8 PERC criteria are negative. Only then can PE be excluded without testing. If any condition is unmet, proceed to D-dimer or imaging.
Disclaimer & References
For Educational Purposes Only. This calculator and the accompanying clinical information are intended as educational tools for healthcare professionals. They do not replace clinical judgement. Results should be interpreted in the full clinical context. Lab reference ranges vary by institution — verify with your own laboratory. Drug dosages should be confirmed against current prescribing information.
References
- Kline JA, Mitchell AM, Kabrhel C, Richman PB, Courtney DM. Clinical criteria to prevent unnecessary diagnostic testing in emergency department patients with suspected pulmonary embolism. J Thromb Haemost. 2004;2(8):1247–1255. DOI: 10.1111/j.1538-7836.2004.00790.x
- Kline JA, Courtney DM, Kabrhel C, et al. Prospective multicenter evaluation of the pulmonary embolism rule-out criteria. J Thromb Haemost. 2008;6(5):772–780. DOI: 10.1111/j.1538-7836.2008.02944.x
- Freund Y, Cachanado M, Aubry A, et al. Effect of the Pulmonary Embolism Rule-Out Criteria on subsequent thromboembolic events among low-risk emergency department patients: the PROPER randomized clinical trial. JAMA. 2018;319(6):559–566. DOI: 10.1001/jama.2017.21904
- Singh B, Mommer SK, Erwin PJ, Mascarenhas SS, Parsaik AK. Pulmonary embolism rule-out criteria (PERC) in pulmonary embolism — revisited: a systematic review and meta-analysis. Emerg Med J. 2013;30(9):701–706. DOI: 10.1136/emermed-2012-201730
- Righini M, Van Es J, Den Exter PL, et al. Age-adjusted D-dimer cutoff levels to rule out pulmonary embolism: the ADJUST-PE study. JAMA. 2014;311(11):1117–1124. DOI: 10.1001/jama.2014.2135
- Konstantinides SV, Meyer G, Becattini C, et al. 2019 ESC Guidelines for the diagnosis and management of acute pulmonary embolism. Eur Heart J. 2020;41(4):543–603. DOI: 10.1093/eurheartj/ehz405
- Wells PS, Anderson DR, Rodger M, et al. Excluding pulmonary embolism at the bedside without diagnostic imaging: management of patients with suspected pulmonary embolism presenting to the emergency department by using a simple clinical model and D-dimer. Ann Intern Med. 2001;135(2):98–107. DOI: 10.7326/0003-4819-135-2-200107170-00010
- Raja AS, Greenberg JO, Qaseem A, et al. Evaluation of patients with suspected acute pulmonary embolism: best practice advice from the Clinical Guidelines Committee of the American College of Physicians. Ann Intern Med. 2015;163(9):701–711. DOI: 10.7326/M14-1772
- Wolf SJ, McCubbin TR, Feldhaus KM, Smithline HA, Bitterman RA. Prospective validation of Wells criteria in the evaluation of patients with suspected pulmonary embolism. Ann Emerg Med. 2004;44(5):503–510. DOI: 10.1016/j.annemergmed.2004.04.002