The PERC rule is a checklist that, when every item is negative in a patient already thought to be low-risk, lets clinicians reasonably rule out PE without further testing.
PERC applies only to patients already judged low pretest probability. Check every criterion that is present. If none are present (score 0), PERC is negative and PE is effectively excluded. Any positive criterion means PERC cannot rule out PE.
No — it's only validated for patients a clinician has already judged to be low pretest probability for PE, not as a first-line screening tool for everyone.
If all eight criteria are negative in an already-low-risk patient, the probability of PE is low enough that further workup (including D-dimer) is often unnecessary.
PERC is 'positive' and cannot rule out PE; the workup continues based on pretest probability, typically with a D-dimer or imaging.
Both are independent PE risk factors; older age and tachycardia raise the probability enough that the rule can no longer safely exclude PE without testing.