Performance of helical computed tomography in unselected outpatients with suspected pulmonary embolism

Background: Helical computed tomography (CT) is commonly used to diagnose p ulmonary embolism, although its operating characteristics have been insuffi ciently evaluated. Objective: To assess the sensitivity and specificity of helical CT in suspe cted pulmonary embolism. Design: observational study. Setting: Emergency department of a teaching and community hospital. Patients: 299 patients with clinically suspected pulmonary embolism and a p lasma D-dimer level greater than 500 mug/L. Intervention: Pulmonary embolism was established by using a validated algor ithm that included clinical assessment, lower-limb compression ultrasonogra phy, lung scanning, and pulmonary angiography. Measurements: Sensitivity, specificity, and likelihood ratios of helical CT and interobserver agreement. Helical CT scans were withheld from clinician s and were read 3 months after acquisition by radiologists blinded to all c linical data. Results: 118 patients (39%) had pulmonary embolism, In 12 patients (4%), 2 of whom had pulmonary embolism, results of helical CT were inconclusive, Fo r patients with conclusive results, sensitivity of helical CT was 70% (95% CI, 62% to 78%) and specificity was 91% (CI, 86% to 95%), Interobserver agr eement was high (kappa = 0.823 to 0,902). The false-negative rate was lower for helical CT used after initial negative results on ultrasonography than for helical CT alone (21% vs, 30%). Use of helical CT after normal results on initial ultrasonography and nondiagnostic results on lung scanning had a false-negative rate of only 5% and a false-positive rate of only 7%. Conclusion: Helical CT should not be used alone for suspected pulmonary emb olism but could replace angiography in combined strategies that include ult rasonography and lung scanning.