How to use it
- Decide first whether the clinical suspicion of pulmonary embolism is low. If it is not, stop: the rule is not meant for that patient.
- Tick each criterion that is present.
- Read the result. Zero ticks is PERC negative; one or more is PERC positive.
How the rule works
PERC (Pulmonary Embolism Rule-out Criteria) lists eight findings. Each counts as one, but the rule is all-or-nothing: the answer is negative only when the count is 0.
| Criterion | Present if |
|---|---|
| Age | 50 years or older |
| Heart rate | 100/min or more |
| Oxygen saturation | below 95% on room air |
| Leg | swelling of one leg |
| Hemoptysis | any coughing up of blood |
| Recent surgery or trauma | within 4 weeks, needing general anesthesia |
| Previous VTE | pulmonary embolism or deep vein thrombosis |
| Hormones | oral contraceptives, hormone replacement or estrogen |
Worked example
A 34-year-old with sharp chest pain after a long run, heart rate 88, saturation 98%, no leg swelling, no blood in the sputum, no recent operation, no past clots and no hormonal medication: 0 criteria, PERC negative. If the clinician’s suspicion was low to begin with, PE is ruled out without a D-dimer.
The same patient taking the combined pill: 1 criterion, PERC positive, so the rule cannot be used and a D-dimer is the usual next step.
Common mistakes
- Applying PERC to a patient whose suspicion is moderate or high. The rule is a filter for low-risk patients only.
- Reading saturation after oxygen has been started; the criterion is on room air.
- Treating PERC positive as a diagnosis. It only sends the patient on to the next test.
- This page is for information. Chest pain or sudden breathlessness needs urgent medical care.