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Clinical reference calculator · GeraClinic

PERC Rule Calculator (Pulmonary Embolism Rule-out Criteria)

The PERC rule (Pulmonary Embolism Rule-out Criteria) is an eight-item decision rule used in emergency patients already judged to be at low risk of pulmonary embolism: if all eight criteria are absent, PE can be excluded without D-dimer or imaging. Any positive criterion makes the rule positive.

Quick answer

The PERC rule is applied only when a clinician already judges pulmonary embolism to be low-risk (under about 15%). If all eight criteria are absent — age under 50, pulse under 100, oxygen saturation 95% or more, no unilateral leg swelling, no haemoptysis, no recent surgery or trauma, no prior PE/DVT and no oestrogen use — PE can be excluded without D-dimer or imaging. Any positive criterion makes PERC positive.

Pretest probability

PERC criteria (any present = positive)

PERC +

PERC does not apply — pretest probability is not low (≥ 15%)

PERC does not apply — pretest probability is not low (≥ 15%).

PERC only applies when the pretest probability is already low. If it is, and all eight criteria are absent, the probability of pulmonary embolism is low enough (about 1% in validation studies) to avoid D-dimer and imaging. Any positive criterion — or a pretest probability that is not low — means PERC cannot exclude PE, and further testing is needed. This is a decision aid, not a diagnosis.

How to use the PERC calculator

  1. 1Confirm low pretest probability. First confirm the clinician’s pretest probability of PE is already low (under ~15%). PERC is not valid for moderate- or high-risk patients.
  2. 2Check each of the eight criteria. Tick any criterion that is present: age 50 or over, pulse 100 or more, SaO₂ under 95% on room air, unilateral leg swelling, haemoptysis, recent surgery or trauma, prior PE/DVT, or exogenous oestrogen use.
  3. 3Read the result. If pretest probability is low and none of the eight criteria are present, PERC is negative and PE can be excluded without further testing. Any positive criterion means PERC cannot rule out PE.

Medical disclaimer: This is general health information, not medical advice. It does not diagnose or treat any condition, and the results are estimates based on public reference formulas. Always consult a qualified doctor about your individual health. If you think you may have a medical emergency, contact your local emergency services immediately.

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Frequently asked questions

When can I use the PERC rule?⌄

PERC is valid only when a clinician’s gestalt pretest probability of pulmonary embolism is already low (about 15% or less). If the suspicion of PE is moderate or high, PERC should not be applied — further testing such as D-dimer or imaging is needed instead.

What does a PERC-negative result mean?⌄

In a low-risk patient where all eight criteria are absent, PERC-negative means the probability of PE is low enough (around 1% in validation studies) to forgo D-dimer and imaging. It does not prove there is no clot; it supports a clinical decision.

Why does oestrogen use count in PERC?⌄

Exogenous oestrogen (for example some contraceptives or hormone therapy) raises clotting risk, so its presence is one of the eight criteria that, if positive, stops PE being ruled out by PERC alone.

Is my data stored?⌄

No. The calculator runs entirely in your browser; nothing you enter is sent to a server.

Sources & validation

This calculator reproduces the published PERC score, validated for excluding pulmonary embolism in emergency patients with an already-low pretest probability (<15%).

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