PERC Rule Calculator (Pulmonary Embolism Rule-out Criteria)

Background.

The PERC rule — the Pulmonary Embolism Rule-out Criteria — is a set of eight bedside criteria that, when all are satisfied in a patient whose pretest probability of pulmonary embolism has already been judged low, allows PE to be excluded without a D-dimer and without imaging. It is a clinical decision aid for clinicians, it is not a diagnosis, and its validity depends entirely on a judgement made before you open it.

That precondition is the whole rule. PERC was derived and validated in emergency-department patients in whom the clinician's gestalt probability of PE was under 15 percent. Applying it to a patient with a moderate or high pretest probability is not a shortcut; it is a misuse, because the rule's safety comes from the low prior it was built on. This calculator deliberately does not compute or assume the probability for you. It checks the eight criteria and tells you what it found, and the caveat travels with the result rather than sitting in an accordion.

PERC is also not a score. There is no total that gets better or worse. All eight criteria must be satisfied for a PERC-negative result, and one failure is as decisive as eight. The count this page reports is a count of criteria not satisfied, printed so you can see immediately what is standing in the way — but two failures do not mean twice the risk of one, and a PERC-positive result does not mean PE is likely. It means the rule cannot exclude PE, and the diagnostic pathway continues with a D-dimer or imaging as your local protocol directs.

The eight criteria are: age under 50; heart rate under 100; oxygen saturation of 95 percent or more on room air; no haemoptysis; no exogenous oestrogen use; no prior DVT or PE; no unilateral leg swelling; and no surgery or trauma requiring hospitalisation in the past four weeks. Three are measured and five are history or examination findings.

The three measured criteria do not all use the same comparison, and this is where implementations quietly diverge. Age must be strictly under 50, so a patient who has just turned 50 fails. Heart rate must be strictly under 100, so a rate of exactly 100 fails. Oxygen saturation must be 95 percent or more, so exactly 95 passes. Two strict inequalities and one inclusive one. This calculator applies each exactly as published and its tests check the value immediately below, at, and immediately above every threshold.

One practical detail matters more than it looks: the saturation must be measured on room air. A reading of 97 percent on two litres of oxygen does not satisfy the criterion, because the criterion is about the patient's unsupported gas exchange. Entering a supplemented saturation turns a rule-out into a false reassurance, which is the most consequential way this page can be misused.

Finally, scope. PERC was derived and validated in adult emergency-department patients with suspected PE and a low clinical pretest probability. It is not validated in pregnancy, and it is not validated in inpatients or in populations with a high baseline prevalence of PE, where the arithmetic of a rule-out changes. This page reports no miss rate or failure rate, because the published figures depend on the setting's baseline prevalence and quoting one cohort's number as universal would misrepresent it.

What is perc rule calculator?

PERC is a rule-out instrument for pulmonary embolism, derived by Kline and colleagues in 2004 and validated prospectively across multiple centres in 2008. It consists of eight criteria, all of which must be satisfied for the rule to be negative.

Its purpose is narrow and useful: in a low-probability patient, a negative PERC allows the clinician to stop, without a D-dimer. That matters because D-dimer is sensitive but not specific, and in a low-prevalence population a positive D-dimer generates CT scans that were never going to find anything — with the contrast load, radiation and incidental findings that come with them.

The rule sits second in a two-step pathway. Something must establish the pretest probability first: clinician gestalt in the derivation studies, or a structured rule such as the Wells score for PE in many local protocols. Only then does PERC apply. The two are sequential, not interchangeable — a Wells score and a gestalt estimate are different instruments with different operating points, and one cannot be converted into the other.

What PERC is not: a probability score, a severity measure, a substitute for the pretest-probability judgement, or something that can confirm PE. It only ever excludes, and only in the population it was built for.

How to use this calculator.

1. 01Make the pretest-probability judgement first, independently of this page. PERC applies only when you have already judged the probability of PE to be low.
2. 02Enter the age, the measured heart rate, and the oxygen saturation on room air. A saturation recorded on supplemental oxygen cannot satisfy the criterion.
3. 03Answer the five history and examination items. Each is satisfied only when the answer is No.
4. 04Read the verdict. PERC negative requires all eight criteria; a single failure makes it PERC positive.
5. 05Do not read the count as a risk score. Two failures do not mean twice the risk of one — the count is shown only so you can see which criteria are in the way.
6. 06If the rule is positive, continue down your local diagnostic pathway with a D-dimer or imaging. PERC positive is not a diagnosis and does not mean PE is likely.
7. 07Do not apply the rule in pregnancy, in inpatients, or in a population with a high baseline prevalence of PE.

The formula.

PERC is not an arithmetic score; it is a conjunction. The rule is negative if and only if all eight criteria are satisfied simultaneously, and positive otherwise. This calculator reports the number of criteria not satisfied purely as a diagnostic aid for the user — so you can see what is blocking a negative result — and never as a graded risk.

The three measured criteria are: age strictly under 50 years; heart rate strictly under 100 beats per minute; and oxygen saturation of 95 percent or more, measured on room air. The five history and examination criteria are satisfied when the answer is no: no haemoptysis, no exogenous oestrogen use, no prior DVT or PE, no unilateral leg swelling, and no surgery or trauma requiring hospitalisation in the past four weeks.

Rounding stage: there is none. The count is an exact integer between 0 and 8, and each measured value is compared against its threshold exactly as entered. An oxygen saturation of 94.9 percent fails; it is not rounded up to 95. An age of 49.9 passes and 50 fails.

The comparison directions are deliberately asymmetric and are reproduced here as published. Age and heart rate use strict inequalities — under 50 and under 100 — so exactly 50 and exactly 100 both fail. Oxygen saturation uses an inclusive one — 95 or more — so exactly 95 passes. Getting one of these backwards changes the verdict for real patients sitting exactly on a threshold, which is why each is tested immediately below, at, and immediately above its cut.

Worked through the example on this page: a 44-year-old with a heart rate of 88, an oxygen saturation of 97 percent on room air, and no haemoptysis, no oestrogen use, no prior venous thromboembolism, no unilateral leg swelling and no recent surgery or trauma satisfies all eight criteria. The count of failures is 0 and the verdict is PERC negative. Change the age to 52 and add a combined oral contraceptive, and two criteria fail — age and oestrogen use — giving a count of 2 and a PERC-positive verdict. That verdict says the rule cannot exclude PE. It says nothing about how likely PE now is.

A worked example.

Frequently asked questions.

References& sources.

1. [1]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. Derivation of the eight PERC criteria. (Abstract retrieved 2026-07-29; publisher full text paywalled.)
2. [2]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. Prospective multicentre validation; establishes that the rule is validated only in combination with a low clinical pretest probability. (Abstract retrieved 2026-07-29.)
3. [3]Konstantinides SV, Meyer G, Becattini C, et al. 2019 ESC Guidelines for the diagnosis and management of acute pulmonary embolism developed in collaboration with the European Respiratory Society (ERS). Eur Heart J. 2020;41(4):543–603. Consulted for the ordering of clinical probability assessment before any exclusion strategy, and for the observed PE prevalence by probability category. (Retrieved 2026-07-29.)
4. [4]Wells PS, Anderson DR, Rodger M, et al. Derivation of a simple clinical model to categorize patients probability of pulmonary embolism. Thromb Haemost. 2000;83(3):416–420. Cross-referenced because a Wells pretest probability is what many local protocols substitute for the derivation's clinician gestalt — a substitution the PERC studies did not themselves validate. (Abstract retrieved verbatim 2026-07-29.)