PERC rule calculator
Eight criteria that, if all are negative, allow PE to be excluded without any D-dimer or imaging — but only when clinical suspicion is already very low.
What it is used for
The Pulmonary Embolism Rule-out Criteria (PERC) exist to stop the diagnostic cascade before it starts. In a patient whose gestalt probability of PE is already under 15%, a completely negative PERC means the risk of PE is low enough that no further testing is warranted, avoiding a D-dimer that would very likely be a false positive and lead to an unnecessary CTPA. PERC is a rule-out instrument only: a positive criterion never diagnoses PE, it simply means PERC cannot be used and the patient continues down the Wells and D-dimer pathway.
The eight criteria (all must be absent)
| Component | Points |
|---|---|
| Age 50 or over | 1 |
| Heart rate 100 bpm or over | 1 |
| SpO₂ below 95% on room air | 1 |
| Unilateral leg swelling | 1 |
| Haemoptysis | 1 |
| Recent surgery or traumaWithin 4 weeks | 1 |
| Prior DVT or PE | 1 |
| Hormone useCombined oral contraceptive, HRT or testosterone | 1 |
How the score is interpreted
When not to use it
- PERC is only valid where pre-test probability is already low. Applying it to a moderate- or high-suspicion patient is unsafe.
- A positive criterion is not evidence of PE — it only invalidates the rule.
- Not validated in pregnancy, and of limited use in populations with a high baseline PE prevalence.
Related scores
Browse all 51 calculators in the ResusDoc Clinical Scores app →