Helical computed tomography (CT) techniques for the diagnosis of pulmonary embolism have been refined over the past decade. Helical CT is widely used in the diagnosis of pulmonary embolism despite the lack of well-designed trials supporting this approach. Although helical CT correlates well with pulmonary angiography in detecting central emboli, critics argue that it misses more distal embolic events. It is unknown, however, whether distal emboli are clinically significant. If undetected distal emboli are significant, we reasoned that venous thromboembolic events should occur more often in patients with negative helical CT results who were not receiving anticoagulation.
We performed a retrospective analysis of 433 sequential helical CT scans ordered for clinical suspicion of pulmonary embolism from March 9, 1999, until April 30, 2002. We excluded 119 studies (27%) that were positive for pulmonary embolism, then excluded 57 others of patients who had received anticoagulation throughout the study period. We then contacted patients and families, and reviewed hospital records and death summaries to determine whether the patients had developed any venous thromboembolic events during the 3-month period following their negative helical CT.
Follow-up was completed on 239 (98.4%) of 243 patients. Venous thromboembolic events developed in 4 (1.7%; 95% confidence interval, 0.0%-3.2%). In the 3-month follow-up period, 33 patients died, 1 of a probable pulmonary embolism (0.4% of the study group; 95% confidence interval, 0.0%-1.2%).
Our data support helical CT as a safe, definitive, minimally invasive test that is associated with a low 3-month risk of venous thromboembolism, and may be comparable to results of negative pulmonary angiography or low-probabililty ventilation-perfusion scan.