PERC Rule Calculator: Pulmonary Embolism Rule-out Criteria
Apply the eight PERC criteria (age ≥ 50, heart rate ≥ 100, SpO₂ < 95%, leg swelling, haemoptysis, recent surgery or trauma, prior VTE, hormone use) to a patient with low pre-test probability: see whether pulmonary embolism can be ruled out without a D-dimer, which criteria are positive and the next step.
How it works
PERC negative = clinician gestalt pre-test probability below 15% AND none of: age ≥ 50 · heart rate ≥ 100 · SpO₂ on room air < 95% · unilateral leg swelling · haemoptysis · surgery or trauma ≤ 4 weeks ago needing general anaesthesia · prior PE or DVT · hormone use (oral contraceptives, hormone replacement or oestrogen)
The Pulmonary Embolism Rule-out Criteria (Kline, 2004) let a clinician stop the work-up for pulmonary embolism (PE) without any blood test. They exist because the D-dimer is often falsely positive, and each false positive can lead to a CT pulmonary angiogram, contrast and radiation that do the patient more harm than good when the chance of PE is already tiny. The rule has two gates. First, the clinician's own overall estimate (gestalt) of PE must be below 15%. Second, all eight items must be absent: age 50 or over, heart rate 100 or more, room-air oxygen saturation below 95%, swelling of one leg, haemoptysis, surgery or trauma needing general anaesthesia in the previous 4 weeks, a previous PE or DVT, and use of oral contraceptives, hormone replacement or oestrogen. If both gates are passed the result is PERC negative and PE is considered ruled out, because the remaining probability is below about 2%, the level at which testing causes more harm than it prevents. If even one item is present, PERC cannot rule out PE and the usual next step is a D-dimer (age-adjusted after 50), with CT only if it is positive. PERC was validated prospectively in more than 8,000 emergency patients across 13 US departments (Kline, 2008), and the PROPER cluster-randomised trial in 14 French emergency departments (Freund, JAMA 2018) found a 3-month missed-PE rate of 0.1% with PERC versus 0% with usual care, while CT use fell from 23% to 13%. It is not valid for moderate- or high-probability patients, in pregnancy, or where the local prevalence of PE among tested patients is high.
Worked example
With the defaults: a patient whose clinician judges the pre-test probability as low (below 15%) and who has none of the eight criteria. Step 1: gestalt is low, so PERC applies. Step 2: 0 of 8 criteria are present. Result: "PERC negative: PE ruled out without further testing", residual risk below about 2%, and no D-dimer or imaging is needed. If the same patient were 55 years old, 1 criterion would be present, the result would be "PERC positive: 1 criterion present" and the next step would be an age-adjusted D-dimer (cut-off 550 µg/L FEU).
Sources: MDCalc: PERC Rule for Pulmonary Embolism (criteria wording and use only when gestalt < 15%) · WikEM: PERC rule (criteria, Kline 2004 derivation and Kline 2008 validation) · Freund Y et al. Effect of the PERC rule on subsequent thromboembolic events (PROPER trial), JAMA 2018
Frequently asked questions
What are the 8 PERC criteria?
Age 50 or older; heart rate 100 or more; oxygen saturation on room air below 95%; unilateral leg swelling; haemoptysis; surgery or trauma within 4 weeks that needed general anaesthesia; prior pulmonary embolism or DVT; and hormone use (oral contraceptives, hormone replacement or oestrogen). All eight must be absent for PERC to be negative.
When can the PERC rule be used?
Only when the clinician's gestalt pre-test probability of PE is already low, below 15%. PERC is a rule-out tool for low-risk patients; it must not be used for patients with moderate or high suspicion, and it is not a substitute for clinical judgement.
Does the PERC rule replace the Wells score or the D-dimer?
No. Gestalt or the Wells score sets the pre-test probability; PERC decides whether a low-probability patient needs any test at all. If PERC is positive, the next step is a D-dimer, and imaging follows only if the D-dimer is positive.
What if only one PERC criterion is positive?
Then PERC is positive and cannot rule out PE. One criterion is enough; there is no partial credit. Order a D-dimer (age-adjusted after 50: age × 10 µg/L FEU) and image only if it is above the cut-off.
Can PERC be used in pregnancy?
No. Pregnant patients were not part of the derivation or validation, and pregnancy raises the baseline risk of venous thromboembolism. Use a pregnancy-adapted pathway instead.
How safe is a negative PERC?
In the PROPER trial of 1,916 low-risk emergency patients, a PERC-based strategy missed PE in 0.1% at 3 months and was non-inferior to usual testing, while cutting CT pulmonary angiography from 23% to 13% and shortening emergency stays.
Related calculators
Clinical decision tool for education and for use by trained clinicians alongside judgement. It is not a diagnosis and not a substitute for medical assessment. PERC applies only when the clinician's own pre-test probability of pulmonary embolism is already low (below 15%); it was not validated in pregnancy, in patients with moderate or high suspicion, or in high-prevalence settings.