About the PERC Rule Calculator
The Pulmonary Embolism Rule-out Criteria (PERC) were derived by Jeffrey Kline and colleagues from 3,148 emergency department patients at ten US hospitals and published in the Journal of Thrombosis and Haemostasis in 2004. The rule identifies patients whose probability of pulmonary embolism (PE) is so low that the risks of further testing, including a D-dimer, outweigh the benefit.
PERC is applied only after the clinician has already judged the patient to be low risk — a gestalt pretest probability below about 15%. If all eight criteria are met (age under 50, pulse under 100, oxygen saturation of 95% or more, no unilateral leg swelling, no haemoptysis, no recent surgery or trauma, no prior PE or DVT and no hormone use), the patient is "PERC negative" and PE is considered ruled out without further testing. If any criterion is not met, the rule cannot be used to exclude PE.
This calculator is for emergency clinicians, primary care and students. It is not intended for patients with moderate or high pretest probability, and it should not be applied where PE prevalence is high. In the validation groups the rule had sensitivities of 96–100% with low specificity, so a PERC-positive result simply means the usual diagnostic pathway applies.
How to use the perc rule calculator
- 1Decide on clinical grounds whether the patient’s pretest probability of PE is below 15%.
- 2Enter age, heart rate and room-air oxygen saturation.
- 3Tick any of: unilateral leg swelling, haemoptysis, recent surgery or trauma, prior VTE, hormone use.
- 4Read the PERC result and the list of any criteria not met.
- 5If PERC is positive or not applicable, follow the standard PE diagnostic pathway.
Formula and method
PERC is a block rule: all eight criteria must be satisfied for the rule to be negative, and failing any single one makes it positive. It was built by logistic regression with backward elimination of 21 candidate variables and calibrated so that, in a low-risk patient, a negative rule pushes the probability of PE below the 1.8% threshold at which D-dimer testing does more harm than good.
The rule assumes the clinician has already estimated a low pretest probability (under about 15%). In the original validation, the rule was 96% sensitive in a low-risk group with 8% PE prevalence (1,427 patients) and 100% sensitive in a very-low-risk group with 2% prevalence (382 patients), but specificity was only 27% and 15%, so most PERC-positive patients do not have PE. PE prevalence among patients meeting all criteria was 1.4% and 0% respectively.
Worked examples
Low-risk 35-year-old meeting all criteria
Age 35, pulse 82, SpO₂ 98% and none of the five history items present: all eight criteria are met in a low-risk patient, so PERC is negative.
Young woman on the combined pill
Everything else is normal, but oestrogen-containing contraception fails one criterion, so PERC cannot exclude PE and further testing such as a D-dimer is needed.
Older patient with tachycardia and low saturation
Age 62, pulse 108 and SpO₂ 93% each fail a criterion, giving 3 of 8 not met. PERC is positive and the standard pathway applies.
Clinician judges risk not low
Even though all eight criteria are met, PERC cannot be used when the clinician’s gestalt probability is 15% or more; use Wells or Geneva scoring instead.
Frequently asked questions
What are the 8 PERC criteria?+
Age under 50, heart rate under 100, oxygen saturation 95% or higher, no unilateral leg swelling, no haemoptysis, no surgery or trauma in the past 4 weeks, no prior PE or DVT, and no hormone use (oral contraceptives, HRT or oestrogen).
When should the PERC rule be used?+
Only in patients the clinician already considers low risk for pulmonary embolism, with a pretest probability under about 15%. It is not for patients with moderate or high suspicion.
What does PERC negative mean?+
A PERC-negative patient meets all eight criteria and has a low gestalt probability. Their risk of PE is below the threshold where D-dimer testing is worthwhile, so PE can be considered ruled out without further tests.
Is PERC the same as the Wells score?+
No. Wells estimates pretest probability of PE, while PERC is a rule-out tool used only after probability has been judged low. They are often used together: low Wells or gestalt first, then PERC.
How accurate is the PERC rule?+
In the 2004 validation it was 96% sensitive in a low-risk group and 100% sensitive in a very-low-risk group, with specificity of 27% and 15%. Later studies and meta-analyses confirm a very low miss rate in low-prevalence settings.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.