Ascites or pleural effusion? CT and ultrasound differentiation

The differentiation of fluid immediately above the diaphragm, i.e., pleural effusion, from subdiaphragmatic fluid, i.e., ascites, can be difficult. Freely mobile pleural effusions are easily proven with decubitus chest films, but loculated subpulmonic effusions can mimic intraabdominal fluid. The simultaneous presence of both ascites and pleural effusion is difficult to identify with plain radiographs. Both computed tomography (CT) and ultrasound (US) can be used to differentiate ascites from pleural effusion. Four criteria have been described to differentiate ascites from pleural effusion by CT. These four signs (the diaphragm sign, the displaced crus sign, the interface sign, and the bare area sign) are reliable when only one abnormal fluid collection is present. When both a pleural effusion and ascites are present, none of these criteria can reliably identify both fluid collections. The combined use of the four criteria, however, leads to a correct identification of abnormal fluid collections in the region of the diaphragm. US examination can differentiate ascites from pleural effusion using three of the above-mentioned signs (the diaphragm sign, the displaced crus sign, and the bare area sign). We will review the four signs and discuss the limitation of these signs in clinical practice. A thorough understanding of these CT and US criteria will allow for accurate identification of all juxtadiaphragmatic fluid collections.