Purpose To determine the diagnostic performance of bedside assessment of end-tidal alveolar dead space fraction (ADSF) for pulmonary embolism (PE) and whether the use of additional ADSF assessment following D-dimer assay can improve the diagnostic accuracy in suspected PE patients in the emergency department.

Methods A prospective observational study of 112 consecutive adult patients suspected of PE of whom 102 were eligible for analysis. ADSF was calculated using arterial carbon dioxide and end-tidal carbon dioxide. An ADSF less than 0.2 was considered normal.

Results PE was confirmed in 11 (10.8%) of 102 patients. D-dimer assay alone as a reference standard test for PE had a sensitivity of 100%, specificity of 38.5% and false negativity of 0%. Area under the receiver-operator characteristic curve for the diagnosis of PE using ADSF values alone was 0.894, Sensitivity, specificity and false negativity for the combined results of a positive D-dimer test and abnormal ADSF were 100%, 78.0% and 0% for the presence of PE, respectively. Of 65 patients with a low or intermediate clinical probability and a positive D-dimer assay, 36 (55.4%) patients displayed normal ADSF and had no PE.

Conclusions By itself ADSF assessment performed well in diagnosis of PE. The combined result of a positive D-dimer and abnormal ADSF increased the specificity for diagnosing PE compared with the D-dimer test alone. The use of additional bedside ADSF assessment following a positive D-dimer test may reduce the need for further imaging studies to detect PE in patients with a low or intermediate clinical probability.