The criteria are: Age < 50 years HR < 100 Oxygen saturation ≥ 95% No prior deep venous thrombosis or pulmonary embolism No unilateral leg swelling No estrogen use No hemoptysis No surgery or trauma requiring hospitalization within the past 4 weeks Use of the PERC rule has been recommended as a way to decrease rates of testing for PE with conventional testing using D-dimer, but with similar rates of sensitivity and negative predictive values.