Pulmonary embolism at follow-up outpatient CT pulmonary angiography: implications on patient risk stratification

The purpose of this study was to determine the prevalence of pulmonary embolism in outpatients who return to care with clinical suspicion of pulmonary embolism and are evaluated by computed tomography pulmonary angiogram (CTPA) after an initial CTPA was negative for pulmonary embolism within the preceding 12 months. Following institutional review board approval, we performed a retrospective review of all CTPAs performed at our institution from June 2006 through June 2009. One hundred and seventy-two outpatients [102 women; mean age 56.7±18.8 (SD)] with an initial CTPA that was negative for pulmonary embolism and a subsequent CTPA within 12 months of their initial study were included in our analysis. Each patient's CTPA was assessed for evidence of pulmonary embolism and their electronic medical records (EMR) reviewed for the presence of risk factors associated with venous thromboembolism (VTE). Fisher exact test (two-tailed) analysis was used to assess whether thromboembolic risk factors had an effect on developing pulmonary embolism after an initial negative CTPA. CTPAs were negative for pulmonary embolism in 165 (96%) of 172 outpatients who returned to care within 12 months after an initial negative CTPA. Eighty-five (49.4%) of 172 patients had no identified thromboembolic risk factors. In the group with no risk factors none (0%) of 85 patients (P=0.028) had pulmonary embolism at the time of repeat CTPA. This may help appropriately triage patients evaluated for pulmonary embolism and reduce the number of unnecessary CTPAs.