Imaging of pleural fluid in healthy individuals

The aim of the present study was to determine the ability of radiography and ultrasonography to detect normal pleural fluid in healthy individuals and to assess the frequency of this finding.

Chest ultrasonography of both pleural spaces was performed in a group of 106 healthy volunteers to identify pleural fluid, first in the lateral decubitus position and than leaning on one elbow. Posteroanterior (PA) and lateral decubitus expiratory radiography were subsequently performed. An anechoic layer at least 2 mm thick on chest ultrasonography and a density with a horizontal level at least 3 mm in depth on lateral decubitus radiography were taken as positive results.

On ultrasonography the fluid layer with a typical wedge-shaped appearance was visible in the pleural space of 28 of 106 (26%) volunteers, on both sides in 17 of 28 (61%) and unilaterally in 11 of 28 (39%). The mean fluid layer thickness in both positions was 2.84 mm (SD 0.41 mm, range 2.0-4.3 mm). Mean thickness in the decubitus position was found to be significantly larger than in the elbow position (P < 0.01). Lateral decubitus expiratory radiography showed physiological pleural fluid in only one case.

Chest ultrasonography is superior to lateral decubitus expiratory chest radiography for demonstrating small amounts of normal pleural fluid in healthy individuals. A positive result, if detected, should not be taken as a sign of occult thoracic disease.