Fluid analysis is used to help diagnose the cause of accumulation of fluid in the chest cavity (pleural effusion). It is a test that examines a sample of fluid that has collected in the pleural space. This is the space between the lining of the outside of the lungs (pleura) and the wall of the chest. When fluid collects in the pleural space, the condition is called pleural effusion. A procedure called thoracentesis is used to get a sample of pleural fluid.