PERC Calculator: Pulmonary Embolism Rule-Out Criteria
The PERC (Pulmonary Embolism Rule-Out Criteria) rule uses eight yes/no questions to determine whether a low-risk patient can be safely discharged without D-dimer testing or imaging. Answer each criterion for your patient. A score of zero in a patient already judged to have less than 15% pre-test probability means PE can be excluded with fewer than 2% missed diagnoses. Any positive criterion triggers further workup.
What is the PERC rule?
The Pulmonary Embolism Rule-Out Criteria (PERC) rule is an 8-item clinical decision instrument developed by Kline and colleagues and published in 2004, with a multi-centre validation study in 2008. Its purpose is to allow emergency physicians to exclude pulmonary embolism in patients who are already judged to be at low pre-test risk (below about 15% by clinical gestalt or a Wells score in the low category), without ordering a D-dimer test. This matters because D-dimer is sensitive but not specific: a positive result triggers CT pulmonary angiography in many patients who do not actually have PE, exposing them to radiation and contrast dye. The PERC rule can eliminate that cascade in the right patient.
How is the score calculated?
The physician scores one point for each criterion that is present, giving a total from 0 to 8. The rule is binary in its application: a score of 0 (all criteria absent) in a patient with pre-test probability below 15% is called PERC-negative, and PE may be safely excluded with no further testing. A score of 1 or more (any single criterion present) is called PERC-positive, meaning the rule-out threshold has not been met and D-dimer or imaging must follow. The rule does not stratify beyond PERC-negative versus PERC-positive; a higher score does not directly correspond to a higher PE probability, only to a greater need for investigation.
Who can use the PERC rule?
PERC applies only to adults with a low pre-test probability of PE, typically an emergency department setting where the clinician has already considered the presentation and judged PE risk to be below 15%. It should not be applied to patients with moderate or high pre-test probability, active malignancy, thrombophilia, pregnancy, or to those who were not properly assessed by clinical gestalt or a validated scoring tool (such as the Wells criteria) first. It is also not validated for use in children or in patients with known chronic thromboembolic disease. The rule cannot be used if the O2 saturation criterion cannot be assessed reliably, for example if the patient is receiving supplemental oxygen.
Understanding the individual criteria
Age 50 or older increases VTE risk because clotting factor levels and endothelial function change with age. Tachycardia (heart rate 100 bpm or higher) may be a compensatory response to hypoxia or haemodynamic compromise from PE, though it has many other causes. Oxygen saturation below 95% on room air reflects impaired gas exchange from vascular occlusion. Unilateral leg swelling suggests proximal DVT, which is the most common source of PE. Hemoptysis points to pulmonary infarction, a complication of PE. Recent surgery or major trauma within four weeks dramatically raises VTE risk through immobilisation and pro-coagulant activation. A history of prior PE or DVT marks a patient as inherently higher risk. Exogenous estrogen (oral contraceptives, hormone replacement therapy) raises the VTE risk three- to five-fold by altering coagulation proteins.
PERC Rule Criteria Reference
| Criterion | Threshold / Definition | Clinical note |
|---|---|---|
| Age | 50 years or older | Lower cut-off than many other risk tools |
| Heart rate | 100 bpm or higher | Beta-blockers may mask tachycardia |
| O2 saturation | Below 95% on room air | Cannot apply if on supplemental O2 |
| Leg swelling | Unilateral | Suggests concurrent DVT |
| Hemoptysis | Any blood-tinged sputum | Even trace amounts count |
| Surgery/trauma | Within past 4 weeks | Requiring general or regional anaesthesia |
| Prior PE/DVT | Any confirmed history | Even remote history is positive |
| Estrogen use | OCP, HRT, or other exogenous estrogen | Increases VTE risk 3- to 5-fold |
All 8 criteria must be absent (score = 0) in a pre-test low-risk patient for PERC rule-out. A single positive criterion requires further workup.
Frequently asked questions
Can I use the PERC rule for every patient with chest pain or shortness of breath?
No. PERC applies only when the clinician has already assessed the patient as low risk for PE, typically a pre-test probability below 15% by clinical gestalt or by a validated tool like the Wells score. Applying PERC to moderate- or high-risk patients can cause missed diagnoses: the rule was designed as a rule-out instrument for those who are already unlikely to have PE, not as a screening test.
What does a PERC-negative result mean in practice?
A score of zero, combined with a pre-test probability below 15%, means the post-test probability of PE is below 2%, which is comparable to the background risk the patient would accept for many routine medical decisions. In that context, guidelines support forgoing D-dimer testing and imaging entirely. However, the result is not a guarantee: clinicians must monitor for clinical deterioration and reassess if the picture changes.
What happens if only one PERC criterion is positive?
Any single positive criterion makes the patient PERC-positive. The recommended next step is a high-sensitivity D-dimer assay. If the D-dimer is negative and the Wells score is low, PE can generally be excluded. If the D-dimer is elevated, or if the clinical picture suggests moderate to high suspicion, CT pulmonary angiography or a V/Q scan is warranted.
Why is the age cut-off 50 and not 65?
The original Kline et al. study derived and validated the criteria empirically from patient data; the cut-off that best discriminated low-risk from higher-risk patients in their dataset was 50, not 65. Using 65 would exclude fewer patients from further testing and reduce the rule's efficiency. Some later guidelines use an age-adjusted D-dimer strategy (age x 10 mcg/L in patients over 50), which complements rather than replaces PERC.
Does a PERC-negative result mean the patient definitely does not have PE?
No clinical decision rule has 100% sensitivity. The pooled data from the multi-centre PERC validation study show a miss rate below 2% in low-risk patients, which the original authors and major guidelines consider an acceptable threshold for rule-out. That said, PERC is a decision aid, not a replacement for clinical judgment. If there are compelling clinical reasons to suspect PE even with a PERC score of zero, additional testing remains appropriate.
Can the PERC rule be used in patients on oxygen supplementation?
The oxygen saturation criterion requires a reliable room-air reading. If a patient is on supplemental oxygen and you cannot obtain a trustworthy room-air saturation, the criterion cannot be scored accurately. In that case, the PERC rule should not be applied, because one criterion is uninterpretable.