PERC Rule Calculator for Pulmonary Embolism
Apply the PERC rule — the Pulmonary Embolism Rule-out Criteria — to decide whether a patient you already judge to be at low risk of PE can be cleared without any further testing. PERC is eight “no” questions: if every one is negative and your clinical gestalt puts the risk of PE under about 15%, the rule lets you rule PE out without a D-dimer. If any single item is positive, PERC cannot exclude PE.
About the PERC rule
The Pulmonary Embolism Rule-out Criteria let a clinician exclude PE in a low-risk patient without further testing when all eight criteria are negative. It exists to solve a specific problem: D-dimer is so often falsely positive that testing every low-risk patient sends many with no PE on to CT scans they do not need. PERC identifies the subset of low-risk patients in whom even a D-dimer can safely be skipped.
Across pooled studies PERC is about 97% sensitive but only about 22% specific — by design. A rule-out tool is built to almost never miss the disease, accepting many false positives in return. Applied to a genuinely low-risk patient (pre-test probability under roughly 15%), a fully negative PERC drops the probability of PE below about 2%, the point at which the harms of further testing outweigh the harm of stopping. That 2% figure is the whole basis of the rule: it does not claim zero risk, only a risk low enough that testing does more harm than good.
Worked example. A 34-year-old with pleuritic chest pain, a pulse of 88, oxygen saturation of 99%, no leg swelling, no haemoptysis, no recent surgery, no prior clot and no oestrogen use — whom you already judge low risk — answers "no" to all eight. PERC is negative, and PE can be set aside without a D-dimer. Change the pulse to 104 and one criterion is positive: PERC no longer applies, and a D-dimer is the next step.
PERC is a rule-out only. To estimate pre-test probability in the first place, use a scoring tool: the Wells PE score or the revised Geneva score. Source: Kline JA et al., Journal of Thrombosis and Haemostasis 2004; validated by Kline JA et al., 2008; pooled performance from Singh et al., Annals of Emergency Medicine 2012. This calculator is an educational reference, not medical advice.
How this is calculated
| Criterion | Positive if |
|---|---|
| Age | 50 years or older |
| Heart rate | 100 bpm or more |
| Oxygen saturation | Under 95% on room air |
| Unilateral leg swelling | Present |
| Haemoptysis | Coughing up blood |
| Surgery or trauma needing hospital treatment | In the past 4 weeks |
| Prior pulmonary embolism or DVT | Present |
| Oestrogen use | Oral contraceptive or hormone replacement |
PERC is not a point score. Apply it only when clinical judgement already puts the patient at low risk of PE (roughly < 15% pre-test probability). If all eight criteria are negative, PE can be ruled out without further testing (not even a D-dimer). Any single positive criterion means PERC cannot exclude PE, and you proceed with a D-dimer or imaging. Source: Kline JA et al., Journal of Thrombosis and Haemostasis 2004; validated by Kline JA et al., 2008.
Frequently asked questions
What are the eight PERC criteria?
The eight PERC criteria are age 50 or older; heart rate 100 beats per minute or more; oxygen saturation under 95% on room air; unilateral leg swelling; coughing up blood (haemoptysis); surgery or trauma needing hospital treatment in the past four weeks; a previous pulmonary embolism or DVT; and use of oestrogen — oral contraceptives or hormone replacement. The rule is passed only when all eight are answered “no”. Every item comes from the history or a set of vital signs, so PERC needs no blood test or imaging to apply. A single “yes” — an age of 50, a pulse of 104, or an oral contraceptive — makes the rule positive no matter what the other seven show.
How do I use the result?
PERC is applied only after you have already decided, on clinical judgement, that the patient is at low risk of PE — roughly under 15% pre-test probability. If in that low-risk patient every one of the eight criteria is negative, the rule says the risk of missing a PE is low enough that no further testing (not even a D-dimer) is needed. If any criterion is positive, PERC does not apply and you proceed with a D-dimer or imaging.
Why does PERC require the patient to be low-risk first?
PERC was designed and validated as a rule-out tool for patients a clinician has already assessed as low probability. In higher-probability patients a negative PERC does not lower the risk enough to skip testing. Using PERC in a moderate- or high-risk patient can falsely reassure, so the low-risk gestalt is a precondition, not an output of the rule.
Is PERC a score?
PERC is not a score in the usual sense — there is no point total. It is a checklist where the only “pass” is zero positives. A single positive item means PERC is not satisfied. That is why this tool reports how many criteria are met and whether the rule is negative (all clear) or positive (cannot rule out PE by PERC alone).
Who developed PERC?
The PERC rule was derived by Jeffrey Kline and colleagues and published in 2004, then validated in a large multicentre study in 2008. It is widely used in emergency departments to reduce unnecessary D-dimer testing and imaging in low-risk patients. This calculator is an educational reference, not medical advice.
How accurate is the PERC rule?
A systematic review and meta-analysis put the pooled sensitivity of PERC at about 97% and its specificity at about 22%. That combination is exactly what a rule-out tool is meant to have: very few real PEs are missed, at the cost of many false positives. In a patient a clinician has already judged to be low risk (pre-test probability under about 15%), a negative PERC lowers the chance of PE below roughly 2% — the threshold below which the risk of testing outweighs the risk of missing the diagnosis. The low specificity is why PERC is only ever used to rule PE out, never to rule it in.
Does a positive PERC result mean I have a pulmonary embolism?
No — a positive result — any single criterion present — only means PERC cannot rule PE out, so the workup continues with a D-dimer or imaging. Because PERC is deliberately over-sensitive, most people with a positive PERC do not have a PE; the positive simply sends them for the next test rather than clearing them. PERC answers one narrow question — "is it safe to stop here?" — and a "no" is not a diagnosis.
Can the PERC rule be used in pregnancy?
No — PERC has not been validated in pregnant or postpartum patients and is not recommended for them — pregnancy itself raises PE risk and changes several of the criteria, so a negative PERC cannot be trusted to exclude PE. A modified PERC rule for pregnancy is being studied but is not yet established. Pregnant patients with suspected PE are assessed with pathways designed for pregnancy instead.
This is a reference tool, not a diagnosis or medical advice. Results depend on the laboratory, assay and clinical context, and reference intervals vary between labs and by age and sex — always read your result against the range printed on your own report and discuss it with a qualified healthcare professional.
More medical calculators
eGFRCreatinine ClearanceBody Surface AreaCorrected CalciumAnion GapA1C to eAGHOMA-IRQUICKIGMICholesterol RatioMean Arterial PressureQTcWaist-to-Hip RatioWaist-to-Height RatioLean Body MassPack-YearVO₂ MaxMELD ScoreFramingham RiskASCVD RiskCHA₂DS₂-VAScHAS-BLEDWells DVTWells PEMELD 3.0PREVENT RiskCHA₂DS₂-VAGeneva ScoreChild-PughCURB-65Glasgow Coma (GCS)FIB-4 IndexCorrected SodiumSerum OsmolalityAPRIFatty Liver IndexNAFLD Fibrosis ScoreCentor ScoreFeNaqSOFALVH (ECG)ESR (VSH)Thymol TurbidityMaddrey's DFA-a GradientNEWS2ANCWinters' FormulaShock IndexAlvarado ScoreReticulocyte IndexFree Water DeficitPadua ScoreGlasgow-BlatchfordCIWA-ArMMEMaintenance FluidsBishop ScoreVancomycinSteroid ConversionHEART ScorePediatric DoseSOFA ScoreTransferrin SaturationSF Syncope RuleParkland FormulaInsulin CorrectionCaprini ScoreNIHSSPSI / PORTTIMI RiskABCD²BISAPRockallGRACERansonSodium Correction RatePHQ-9GAD-7 Lab unit converters →