A pleural effusion is an abnormal collection of fluid in the pleural space, detectable on X-ray when more than 300ml of fluid is present and clinically apparent when more than 500ml is present.

Clinical signs of a pleural effusion is that the area is stony dull to percussion, breath sounds are reduced or absent and vocal resonance is reduced.

Pleural aspiration of the fluid may be necessary to determine the cause of the pleural effusion.