How this instrument works
PERC counts eight findings: age 50 or older, heart rate 100 or above, oxygen saturation under 95%, a prior DVT or PE, surgery or trauma requiring intubation or hospitalization within the past four weeks, hemoptysis, exogenous estrogen use, and clinical signs of DVT such as unilateral leg swelling. This tool tallies how many of the eight are present, 0 through 8. Unlike a weighted score where the number itself carries meaning at every point, PERC only has one meaningful reading: zero. A count of zero is 'PERC-negative.' A count of one or more is 'PERC-positive,' and the rule stops being useful at that point.
Kline, Mitchell, Kabrhel, Richman, and Courtney derived the rule in a 2004 Journal of Thrombosis and Haemostasis study, testing which combination of bedside findings, when every one was absent, kept the residual chance of missed PE low enough to justify skipping further workup entirely — no D-dimer, no scan. The point of PERC was speed and cost: in a genuinely low-risk patient, ordering a D-dimer that will likely come back mildly elevated for unrelated reasons often triggers a CT scan that was never going to change management, and PERC gives a way to avoid that cascade when it's truly unnecessary.
The rule only works if it's applied to the right patient, and this is the detail most worth getting right. PERC is meant to be used exclusively on patients a clinician has already judged, by clinical gestalt, to be low pretest probability for PE — commonly described as under roughly 15%. It is not a general screening tool for anyone who walks in with chest pain or shortness of breath. Applied to a higher-probability patient, a PERC-negative result doesn't mean what it's supposed to mean, because the rule was never validated on that population. And a PERC-positive result, even just one criterion, doesn't mean PE is likely — it simply means this particular shortcut can't be used here, and the standard workup continues.
- Toggle Age 50 or older and Heart rate 100 or above.
- Toggle SpO2 under 95%.
- Toggle Prior DVT or PE.
- Toggle Surgery or trauma requiring intubation or hospitalization within the past 4 weeks.
- Toggle Hemoptysis and Exogenous estrogen use.
- Toggle Clinical signs of DVT (unilateral leg swelling), then read how many of the 8 criteria are present.
Worked example — one changed detail flips the result
A 42-year-old with none of the 8 findings — normal heart rate, oxygen saturation above 95%, no prior clot, no recent surgery or trauma, no hemoptysis, no estrogen use, no leg swelling — counts 0 of 8. That's a true PERC-negative result, and in a patient a clinician has already judged low probability for PE, this rule alone supports stopping the workup without a D-dimer.
Change only the age to 54 and nothing else in the case: the count moves to 1 of 8, because age 50 or older is now positive. That single change makes the patient PERC-positive. It does not raise the estimated probability of PE — it simply means the all-clear shortcut can no longer be used, and the standard pathway, typically starting with a D-dimer, applies instead.
Questions
Who can PERC actually be applied to?
Only patients a clinician has already judged, by clinical gestalt, to be low pretest probability for pulmonary embolism — commonly described as under roughly 15%. PERC was derived and validated in that specific population. It is not a general screening tool for every patient with chest pain or shortness of breath, and using it on someone with a moderate or high pretest probability misapplies the rule, regardless of how the eight criteria come out.
What does a PERC-positive result mean?
It means at least one of the eight criteria is present, so the rule no longer applies to that patient. It does not mean pulmonary embolism is likely — a single positive criterion, like age 50 or older, carries no probability estimate on its own. It only means this particular shortcut can't be used to skip testing, and the standard workup, typically starting with a D-dimer, continues from there.
What are the eight PERC criteria?
Age 50 or older, heart rate 100 or above, oxygen saturation under 95%, a prior DVT or PE, surgery or trauma within the past four weeks requiring intubation or hospitalization, hemoptysis, exogenous estrogen use, and clinical signs of DVT such as unilateral leg swelling. All eight must be absent for a patient to be PERC-negative.
How reliable is a PERC-negative result?
In the population it was derived and validated for — patients already judged low pretest probability by clinical gestalt — Kline and colleagues' 2004 work found the residual chance of a missed PE after a PERC-negative result low enough to be considered an acceptable trade-off for skipping D-dimer testing and its downstream imaging. That reliability depends entirely on applying the rule to the intended population; it was never tested as a stand-alone screen for everyone with respiratory symptoms.
Does PERC replace a D-dimer test?
Only in the specific patients it's meant for. In a low pretest probability patient who is also PERC-negative, the rule is used instead of ordering a D-dimer, since the added test is unlikely to change management and risks triggering unnecessary imaging from a false positive. In anyone with a higher pretest probability, or anyone who is PERC-positive, a D-dimer or direct imaging remains the standard next step.
Why was PERC developed if D-dimer testing already exists?
Because D-dimer levels rise with age, pregnancy, inflammation, and many unrelated conditions, a low-risk patient can easily generate a mildly elevated D-dimer that leads to a CT scan that was never going to be positive. Kline's team built PERC to identify, using bedside findings alone, which already-low-risk patients could skip that entire cascade — sparing radiation, contrast dye, cost, and emergency-department time without meaningfully increasing missed diagnoses.
References
- Kline, Mitchell, Kabrhel, Richman & Courtney 2004, J Thromb Haemost — PERC rule derivation (PubMed)
- NHLBI — Pulmonary Embolism
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.