Chest sonography versus lateral decubitus radiography in the diagnosis of small pleural effusions.

Purpose: The aim of this prospective study was to assess the value of chest sonography in the radiologic diagnosis of small pleural effusions (relative to expiratory lateral decubitus radiography) and to suggest gray-scale sonographic criteria for detecting the presence of small pleural effusions.

Methods: Patients referred for abdominal or chest sonographic evaluation for various reasons were also examined for sonographic features of pleural effusion from May 1, 1997, until January 31, 2000. Patients who had evidence of small pleural effusions were included. Patients with no such evidence served as a control group. Subsequently, all patients underwent erect posteroanterior and expiratory lateral decubitus chest radiography.

Results: On chest sonography, 52 patients were found to have small pleural effusions. The control group consisted of 17 patients. The mean thickness of the pleural effusion was 9.2 mm on sonography and 7.6 mm on expiratory lateral decubitus radiography (p < 0.01). Compared with radiologic examination, chest sonography had a positive predictive value of 92% in the diagnosis of small pleural effusions in our study population.

Conclusions: Chest sonography showed a high degree of accuracy relative to that of lateral decubitus chest radiography in the diagnosis of small pleural effusions, which appeared as thin (usually 15 mm thick or less) anechoic areas that changed shape with the phases of respiration.