PERC Rule Calculator

Q: What is the PERC rule?

PERC stands for Pulmonary Embolism Rule-out Criteria. It is an eight-item rule that can support ruling out pulmonary embolism only after a patient has already been assessed as having a low pre-test probability.

Q: What makes a PERC result negative?

All eight criteria must be absent. If even one criterion is present, the result is PERC positive and the rule cannot be used to exclude pulmonary embolism.

Q: Can PERC diagnose pulmonary embolism?

No. PERC is a rule-out aid for a narrowly defined low-risk group, not a diagnostic test. It should not be applied on its own to someone with a higher pre-test probability or haemodynamic instability.

Apply the PERC rule (Pulmonary Embolism Rule-out Criteria) — eight criteria that, when all absent, rule out PE in a low pre-test probability patient. Educational tool only.

Tick every criterion that is present

PERC is applied only after the patient has already been assessed as at low pre-test probability of pulmonary embolism.

Age ≥ 50 yearsPERC is validated for use at any age, but age 50 or over counts as a positive criterion.Heart rate ≥ 100 beats/minA resting tachycardia of 100 or more counts as a positive criterion.Oxygen saturation < 95% on room airMeasured by pulse oximetry on room air.Unilateral leg swellingVisible asymmetry or swelling of one leg.HaemoptysisCoughing up blood.Recent surgery or trauma requiring hospitalisation (within 4 weeks)Surgery or trauma requiring hospitalisation during the previous four weeks.Prior pulmonary embolism or deep vein thrombosisA documented previous VTE event.Exogenous oestrogen useCombined oral contraceptive pill, hormone replacement therapy or similar.

Positive criteria

0 of 8

All criteria absent

Rule result

PERC negative

PERC negative — PE ruled out in a low pre-test probability patient

PERC (the Pulmonary Embolism Rule-out Criteria) comes from Kline and colleagues (2004, refined 2008). It is an eight-item rule that applies only to a patient whose pre-test probability of PE has already been judged low: when all eight criteria are absent, PE can be considered excluded without a D-dimer or imaging. Any single positive criterion makes PERC positive, and the patient moves on to D-dimer and/or imaging. PERC is not a diagnostic test, is not validated for use on its own in a patient at higher pre-test probability, and does not apply to haemodynamically unstable patients.

Frequently asked questions

What is the PERC rule?

PERC stands for Pulmonary Embolism Rule-out Criteria. It is an eight-item rule that can support ruling out pulmonary embolism only after a patient has already been assessed as having a low pre-test probability.

What makes a PERC result negative?

All eight criteria must be absent. If even one criterion is present, the result is PERC positive and the rule cannot be used to exclude pulmonary embolism.

Can PERC diagnose pulmonary embolism?

No. PERC is a rule-out aid for a narrowly defined low-risk group, not a diagnostic test. It should not be applied on its own to someone with a higher pre-test probability or haemodynamic instability.

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