PERC Rule Calculator
PERC rule for pulmonary embolism: eight bedside criteria and a negative or positive result.
What it does
The PERC rule, or pulmonary embolism rule-out criteria, is a bedside checklist. It was designed to identify very low-risk patients in whom the chance of PE is low enough that D-dimer testing can be skipped.
The eight criteria are age 50 or older, heart rate 100 or more, oxygen saturation below 95 percent on room air, unilateral leg swelling, hemoptysis, recent surgery or trauma requiring hospitalization within four weeks, prior PE or DVT, and exogenous estrogen use.
PERC is negative only when every criterion is absent. A single positive criterion makes PERC positive and means the rule cannot be used to defer testing. The rule should be applied only after the clinician has estimated pretest probability as low.
The defaults show a 42-year-old with a heart rate of 78, oxygen saturation of 98 percent, and no criteria. All eight are absent, so PERC is negative and the tool notes that D-dimer and imaging can be deferred in a low pretest probability patient.
How to use
- Confirm that the patient has a low clinical pretest probability of PE before using PERC.
- Enter age, heart rate, and room air oxygen saturation.
- Answer the five yes or no criteria: leg swelling, hemoptysis, recent surgery or trauma, prior VTE, and estrogen use.
- Press Calculate to count the criteria present.
- A count of zero is PERC negative; any count above zero is PERC positive.
- Use the result within the local PE pathway, not as a stand-alone clearance.
Formula
- Age
- Positive at 50 years or older.
- Heart rate
- Positive at 100 beats/min or more.
- Oxygen saturation
- Positive below 95 percent on room air.
- Leg swelling
- Positive for unilateral leg swelling.
- Hemoptysis
- Positive when present.
- Recent surgery or trauma
- Positive when requiring hospitalization within 4 weeks.
- Prior VTE
- Positive for prior PE or DVT.
- Estrogen
- Positive for exogenous estrogen use.
Count every criterion that is present. The rule is negative only at a count of zero. Age, heart rate, and oxygen saturation use fixed thresholds; the other five are yes or no items. PERC was derived and validated for low-risk outpatients and is not a stand-alone rule for higher-risk or unstable patients.
Worked example
Example: 42 years old, heart rate 78, saturation 98 percent
Defaults: age 42, heart rate 78/min, oxygen saturation 98 percent on room air, and no other criteria.
- Age 42 is below 50: no point.
- Heart rate 78 is below 100: no point.
- Oxygen saturation 98 percent is at or above 95 percent: no point.
- Leg swelling, hemoptysis, recent surgery or trauma, prior VTE, and estrogen use are all No.
- The criteria count is 0, so PERC is negative.
- In a patient with low pretest probability, PERC negative supports deferring D-dimer and imaging.
Reference table
| Criterion | Positive when |
|---|---|
| Age | 50 years or older |
| Heart rate | 100 beats/min or more |
| Oxygen saturation | Below 95 percent on room air |
| Unilateral leg swelling | Present |
| Hemoptysis | Present |
| Recent surgery or trauma | Hospitalization within 4 weeks |
| Prior PE or DVT | Present |
| Exogenous estrogen | Present |
Limits
- PERC applies only when pretest probability is already low.
- It is not validated for use in unstable patients or those with high clinical suspicion.
- Pregnancy and recent anticoagulation require individualized assessment.
- A positive PERC does not mean the patient has PE; it only means the rule cannot be used to defer testing.
Common mistakes
- Applying PERC without first estimating pretest probability.
- Calling PERC negative when one criterion is present.
- Using oxygen saturation measured on supplemental oxygen.
- Counting age 50 or heart rate 100 as negative when both are positive thresholds.
- Using PERC to avoid evaluation in a patient with high clinical suspicion.
Try it yourself
Quick checks. Answers sit tucked under each question.
A 52-year-old with heart rate 82, saturation 98 percent, and no other criteria: is PERC negative?
No. Age 52 is 50 or older, so one criterion is present and PERC is positive.
A 30-year-old with heart rate 88, saturation 94 percent, and no other criteria: is PERC negative?
No. Oxygen saturation below 95 percent is a positive criterion, so PERC is positive.
A 40-year-old with heart rate 78, saturation 99 percent, and no criteria: what is the result?
0 criteria, PERC negative. In a low pretest probability patient, this supports deferring D-dimer and imaging.
Questions
What does PERC stand for?
Pulmonary embolism rule-out criteria. It is an eight-item bedside rule for low-risk patients.
When is PERC negative?
Only when all eight criteria are absent. A single positive criterion makes the rule positive.
Can PERC rule out PE by itself?
Only in a patient whose pretest probability is already low. It is not a stand-alone test and does not replace clinical judgment.
Does PERC replace D-dimer testing?
In a low-risk patient with a negative PERC, the pathway may allow D-dimer testing to be deferred. Local protocols determine the exact sequence.
Can PERC be used in pregnancy?
Pregnancy requires a dedicated PE pathway. PERC was not designed for pregnancy and should not be applied without specific guidance.
What if the patient has a high pretest probability?
Do not use PERC. Proceed with the diagnostic pathway for PE, which usually includes imaging.