PERC Rule Calculator
Tick each criterion that is satisfied. If all eight are met in a patient whose clinical suspicion is already low, PERC can exclude pulmonary embolism without D-dimer or imaging.
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What PERC Is For
The Pulmonary Embolism Rule-out Criteria are a set of eight findings that, when all are absent, identify patients whose probability of pulmonary embolism is so low that testing can be skipped altogether. PERC is a rule-out tool: it works only when the clinician’s suspicion is already low, and it says nothing about what to do when suspicion is moderate or high.
The Eight Criteria
- Age under 50
- Heart rate under 100 beats per minute
- Oxygen saturation 95 % or more on room air
- No haemoptysis
- No oestrogen use
- No prior deep vein thrombosis or pulmonary embolism
- No unilateral leg swelling
- No surgery or trauma requiring hospitalisation within the past four weeks
If every criterion is satisfied, PERC is negative and pulmonary embolism can be excluded without D-dimer or imaging. If even one is not satisfied, PERC is positive and cannot be used to rule out the diagnosis.
How PERC Fits With the Wells Score
The two tools answer different questions and are used in sequence. A widely used pathway is:
- Assess clinical suspicion first. If it is low, apply PERC.
- PERC negative — no D-dimer, no imaging; look for an alternative explanation.
- PERC positive, or suspicion not low — use a structured score such as the Wells score, then age-adjusted D-dimer in the “unlikely” group or imaging in the “likely” group.
In the original validation the combination of low clinical probability and a negative PERC carried a failure rate under about 2 % for subsequent thromboembolism, which is comparable to the miss rate of a negative D-dimer-based strategy.
Limitations
- It is not a rule-in tool and must never be applied to a patient whose clinical suspicion is moderate or high.
- Maximum utility is in younger, lower-risk patients, because age 50 or more alone makes PERC positive.
- Pregnancy is excluded from the original derivation; specific pathways apply, although PERC has been studied with caution in pregnancy.
- Interpretation of “low suspicion” is subjective, and a negative PERC does not overrule a clinician’s concern.
- It cannot distinguish PE from other serious causes of chest pain, breathlessness or tachycardia.
Frequently Asked Questions
Can PERC be used on its own to send a patient home?
Only as part of a full assessment in a patient whose clinical suspicion is genuinely low. PERC excludes PE, not the other conditions that cause chest pain, hypoxia or tachycardia.
What if PERC is positive?
Nothing is excluded. Move to a structured probability assessment such as the Wells score and follow the D-dimer or imaging pathway that applies to your patient.
Why does age above 50 make PERC positive immediately?
Because age is itself a risk factor for thromboembolism, and the criterion was designed to keep the tool restricted to genuinely low-risk patients.