A computed tomography (CT) measurement of 2.5 cm for size of a pleural effusion associated with pneumonia correlates to a lateral decubitus measurement of 1 cm and is useful for determining the need for thoracentesis.
Source: Moffett BK, Panchabhai TS, Anaya E, et al. Computed tomography measurements of parapneumonic effusion indicative of thoracentesis. Eur Resp J. 2011;38:1406–1411.