π« PERC Rule Calculator
Rule out PE without testing, when the rule fits.
For informational purposes only, not medical advice. Consult a healthcare professional.
The Pulmonary Embolism Rule-out Criteria, or PERC, identify emergency patients in whom PE is so unlikely that no further testing is needed. All eight criteria must be absent: under 50, heart rate under 100, oxygen saturation 95 or above, no leg swelling, no hemoptysis, no recent surgery or trauma, no prior VTE, and no hormone use. Crucially, the rule only applies after the clinician judges pretest probability low. In validation studies, PERC-negative low-risk patients had a PE rate under 1 percent at follow-up. This calculator applies the rule for education and clinical reference.
How to Use This Calculator
This rule only applies when the clinician has already judged the patient low risk for PE. Check each of the 8 criteria that is present.
Confirm low pretest probability first, usually with a Wells score of low risk or gestalt. PERC never applies to moderate or high-risk patients.
Check each criterion present. All 8 must be absent to rule out PE; a single checked box means the rule cannot rule out.
Read the result. PERC-negative means PE is ruled out without D-dimer or CT; PERC-positive means testing proceeds, usually starting with D-dimer.
How the Math Works
Each of the 8 items scores 1 point when present. A total of 0 with low pretest probability is PERC-negative: PE ruled out, no D-dimer, no CT. A total of 1 or more is PERC-positive: the rule cannot rule out, and workup continues, typically with D-dimer first. The rule was derived by Kline and colleagues and validated in large emergency department cohorts. Its power comes from combining individually weak predictors: each alone means little, but all eight absent together define a group whose PE risk falls below the testing threshold where further workup would cause more harm than good.
Worked Example
A 42-year-old with pleuritic chest pain, heart rate 88, oxygen 98 percent, no leg swelling, no hemoptysis, no recent surgery, no prior clots, and no hormone use scores 0 points. With a low pretest probability assessment, the calculator returns PERC-negative: no D-dimer, no CT, with routine follow-up. Change one fact, say a heart rate of 105, and the rule no longer applies.
Reading Your Result
PERC-negative is a stopping rule: stop testing for PE and look elsewhere for the symptoms. It is not a diagnosis of anything else, and the patient still needs evaluation of the actual complaint. PERC-positive is not a PE diagnosis either; it simply means the workup continues down the standard pathway. The most common misuse is applying PERC without the low-risk precondition, which the validation studies do not support.
| PERC result | Meaning | Next step |
|---|---|---|
| 0, low pretest probability | PE ruled out | No D-dimer, no CT |
| 1 or more | Cannot rule out | D-dimer, then CT if positive |
Limitations
PERC applies only to emergency department patients with low pretest probability and symptoms suggesting PE; it is not validated for inpatients, pregnant patients in all studies, or high-risk patients. Pretest probability assessment itself requires clinical judgment or a Wells score. This is a clinical decision aid for trained professionals and students, not a self-diagnosis tool.
Related Calculators
Related tools: revised Geneva score for PE Padua VTE risk score DVT risk: Padua score.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is the PERC rule?
Eight clinical criteria that, when all absent in a low-risk emergency patient, rule out pulmonary embolism without blood tests or imaging. Derived and validated by Kline and colleagues, it safely reduces unnecessary CT scans, which carry radiation and contrast risks.
What are the 8 PERC criteria?
Age under 50, heart rate under 100, oxygen saturation 95 percent or higher on room air, no unilateral leg swelling, no hemoptysis, no surgery or trauma in the past 4 weeks, no prior VTE, and no estrogen hormone use. All eight must be absent.
Who can the PERC rule be applied to?
Emergency department patients whose clinician judges the pretest probability of PE to be low, typically under 15 percent, often formalized with a low Wells score. It is not for admitted inpatients, not for moderate or high-risk patients, and gestalt low risk is the entry ticket.
What does PERC-negative mean?
That PE is ruled out and no further PE testing is needed: no D-dimer, no CT angiography. Follow-up studies show under 1 percent of such patients develop VTE in the next 45 days. The presenting symptoms still need their own evaluation.
What if one criterion is positive?
Then the rule cannot rule out PE, and testing proceeds, usually with a D-dimer first and CT if the D-dimer is positive or the patient is high risk. A single positive criterion is common and does not diagnose PE; it just keeps the workup open.
How is PERC different from Wells score?
Wells estimates pretest probability as low, moderate, or high; PERC then rules out PE within the low-probability group. They are sequential tools: Wells first to establish low risk, PERC second to see if testing can stop entirely. PERC never replaces the probability assessment.
Why does hormone use matter?
Estrogen-containing contraceptives and hormone therapy raise clotting risk, so their use is one of the eight criteria. Its presence alone makes the patient PERC-positive. Progestin-only methods are generally considered lower risk but the rule uses any hormone use as the criterion.
Can PERC be used in pregnancy?
Cautiously. Pregnancy raises heart rate and was an exclusion in some validation work, though later studies suggest the rule performs reasonably. Pregnant patients with suspected PE need clinician judgment with pregnancy-adapted pathways, not a calculator default.
Does a negative D-dimer do the same job?
D-dimer is the next step when PERC is positive or inapplicable: a negative high-sensitivity D-dimer in low or intermediate risk also rules out PE. PERC advantage is ruling out without any blood test at all, saving time, cost, and venipuncture in the right patients.
What is the miss rate of PERC?
In validation studies, under 1 percent of PERC-negative low-risk patients had VTE at 45-day follow-up, below the accepted 2 percent testing threshold. No rule is perfect, which is why the low-pretest-probability precondition and clinical judgment remain essential.