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The objective of our study was to determine the impact of embedding a pretest probability rule that is required during the computerized physician order-entry (CPOE) process on the appropriateness of CT angiography (CTA) of the pulmonary arteries for the diagnosis of pulmonary embolism (PE) in the emergency department (ED).Data were obtained from the electronic medical records of all adults who visited the ED from October 17, 2010, through October 17, 2012 (n = 96,507). The primary outcome was the appropriateness of pulmonary CTA. Logistic regression was used to test whether rates of appropriate use, overuse, and underuse of pulmonary CTA improved significantly after the implementation of the decision support tool when controlling for other patient characteristics.Pulmonary CTA was appropriately used in 67.2% of patients with a modified Wells score of ≥ 4, a positive d-dimer test result, or both. CTA was overused in 19.3% of patients and underused in 13.5% of patients. Each additional month after the start of the intervention was associated with a 4-percentage point increase in the odds that the modified Wells score would indicate CTA had been used appropriately (odds ratio [OR] = 1.04; 95% CI, 1.01-1.07) and significantly lowered the odds of overuse of CTA (OR = 0.93; 95% CI, 0.90-0.96) based on the modified Wells score. These changes were not associated with any significant alteration in the level of CTA utilization or the positivity rate.The addition of a mandatory field in the CPOE record was associated with a significant improvement in the appropriate ordering of pulmonary CTA but did not change the PE positive rate or CTA utilization. It seems likely that physicians gradually inflated the modified Wells scores in spite of the fact that a threshold modified Wells score was not required to perform pulmonary CTA.