PERC Rule Calculator

The Pulmonary Embolism Rule-out Criteria (PERC) allows clinicians to safely exclude PE in low-risk patients without D-dimer testing, reducing unnecessary investigations.

Important Clinical Disclaimer: The PERC Rule is intended for use by qualified healthcare professionals only. It is for educational and informational purposes and should not replace clinical judgement. Always interpret results alongside the full clinical picture and follow your local guidelines and protocols.
PERC Rule Assessment
Pre-requisite: The PERC Rule should only be applied to patients who are already considered LOW risk for PE based on clinical gestalt (clinician's overall assessment). If PE is more than a "low suspicion" concern, proceed directly to D-dimer/imaging.

Answer each criterion. If ALL are "No", the patient is PERC-negative and PE can be safely ruled out.

Clinical evidence of DVT
Coughing up blood
Within 4 weeks, requiring hospitalisation
History of venous thromboembolism
Oestrogen-containing contraceptives or HRT
About the PERC Rule

The PERC Rule was developed by Kline et al. to identify emergency department patients at very low risk of pulmonary embolism who do not need further testing.

Key Features:
  • 8 simple clinical criteria
  • All criteria must be negative for PE to be ruled out
  • Reduces unnecessary D-dimer testing
  • Avoids radiation exposure from CTPA
  • Sensitivity >97% when applied correctly
Important Considerations:
  • Only apply PERC to patients with low pre-test probability (gestalt <15%)
  • PERC should not be used if clinical suspicion is moderate or high
  • When in doubt, proceed with D-dimer
PE Clinical Pathway
  1. Assess pre-test probability (clinical gestalt or Wells Score)
  2. If LOW probability: Apply PERC Rule
    • PERC negative → PE ruled out, no testing needed
    • PERC positive → Proceed to D-dimer
  3. If MODERATE probability: D-dimer testing
    • Negative D-dimer → PE ruled out
    • Positive D-dimer → Imaging (CTPA)
  4. If HIGH probability: Proceed directly to imaging

PERC Criteria Explained

Criterion Definition Clinical Rationale
Age ≥50 years Patient is 50 years or older PE incidence increases significantly with age
Heart rate ≥100 bpm Tachycardia at time of assessment Compensatory response to hypoxia and reduced cardiac output
SpO2 <95% Oxygen saturation below 95% on room air Indicates V/Q mismatch from PE
Unilateral leg swelling Clinical evidence of deep vein thrombosis Most PEs arise from lower extremity DVT
Haemoptysis Coughing up blood Suggests pulmonary infarction from PE
Recent surgery/trauma Within 4 weeks, requiring hospitalisation Major risk factor for VTE (Virchow's triad)
Prior PE or DVT History of venous thromboembolism Significantly increases recurrence risk
Hormone use Oestrogen-containing OCP or HRT Increases thrombotic risk

Evidence Base

Performance
  • Sensitivity: 97.4% (95% CI: 95.8-98.5%)
  • Specificity: 22.7% (95% CI: 21.6-23.8%)
  • Negative predictive value: 99.5%
  • False negative rate: 1-2% (comparable to negative D-dimer)

In the derivation cohort of 8,138 patients, only 1.0% of PERC-negative patients had PE at 45-day follow-up.

Limitations
  • Only for low-risk patients - must assess gestalt first
  • Lower sensitivity than D-dimer-based algorithms
  • Not validated in inpatient settings
  • European validation shows variable performance
  • UK practice often defaults to D-dimer testing

NICE and BTS guidelines discuss PERC but some UK centres do not routinely use it.

Frequently Asked Questions

PERC stands for Pulmonary Embolism Rule-out Criteria. It is a clinical decision rule designed to safely exclude PE in low-risk emergency department patients without the need for D-dimer or imaging.

Do not use PERC if:

  • Your clinical suspicion (gestalt) for PE is moderate or high
  • The patient is an inpatient (not validated in this setting)
  • You have a high-risk presentation (haemodynamic instability)
  • The patient is pregnant (different approach required)

PERC is only appropriate for patients where you have already determined PE is a "low probability" concern.

If any PERC criterion is positive, the patient is "PERC positive" and cannot have PE ruled out by this rule alone. The next step is typically D-dimer testing:

  • Negative D-dimer → PE ruled out
  • Positive D-dimer → Proceed to imaging (CTPA or V/Q scan)

Consider age-adjusted D-dimer thresholds in patients over 50.

Use of PERC in the UK varies:

  • NICE mentions PERC but does not mandate its use
  • BTS guidelines (2018) suggest it can be used in low-risk patients
  • Many UK centres default to D-dimer testing regardless
  • European validation studies have shown variable performance

Follow your local trust/departmental protocols. If in doubt, proceeding with D-dimer is a reasonable approach.

Both are clinical decision rules for PE, but they serve different purposes:

  • Wells Score for PE: Stratifies pre-test probability (low/moderate/high or PE likely/unlikely). Used to decide whether to proceed with D-dimer or imaging.
  • PERC Rule: Applied only to patients already deemed low-risk. If all 8 criteria are negative, PE can be ruled out without any testing.

Think of it as: Wells first (to determine risk level), then PERC (only if low risk) to see if any testing is needed at all.

Disclaimer

This calculator is provided for educational and informational purposes only. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment.

  • Always seek the advice of a qualified healthcare provider with any questions regarding a medical condition.
  • Never disregard professional medical advice or delay seeking it because of information from this tool.
  • Clinical decision-making should always incorporate the full clinical context, patient preferences, and local protocols.
  • The creators of this tool accept no liability for decisions made based on its output.

If you are a patient: please discuss any results with your healthcare provider. This tool is designed for use by medical professionals and may not be appropriate for self-assessment.

References

  1. Kline JA, et al. (2004). Clinical criteria to prevent unnecessary diagnostic testing in emergency department patients with suspected pulmonary embolism. Journal of Thrombosis and Haemostasis, 2(8), 1247-1255.
  2. Kline JA, et al. (2008). Prospective multicenter evaluation of the pulmonary embolism rule-out criteria. Journal of Thrombosis and Haemostasis, 6(5), 772-780.
  3. Freund Y, et al. (2018). Effect of the Pulmonary Embolism Rule-Out Criteria on Subsequent Thromboembolic Events Among Low-Risk Emergency Department Patients: The PROPER Randomized Clinical Trial. JAMA, 319(6), 559-566.
  4. BTS Guideline (2018). British Thoracic Society guidelines for the initial outpatient management of pulmonary embolism (PE). Thorax, 73(Suppl 2), ii1-ii29.
  5. NICE (2020). Venous thromboembolic diseases: diagnosis, management and thrombophilia testing (NG158). National Institute for Health and Care Excellence.