METHODS: Adult patients who underwent a CTPA for suspicion of PE at initial presentation to the emergency department from January 1, 2014 to March 31, 2015 were retrospectively analyzed. Pregnant women and hemodynamically unstable patients (systolic blood pressure<90) were excluded. The CTPA was considered appropriate if Modified Wells score (MWS) was >4 or any score with positive d-dimer assay (>500). Definitive PE, questionable PE and sub-segmental PE on CTPA were considered positive while inconclusive or definitively negative studies were considered to be negative. Sensitivity, Specificity and negative predictive value (NPV) for D Dimer assay were calculated in low risk (MWS<=4) and high-risk (MWS>4) patient population.