Pleural effusion is a fluid build up in the pleural space between the lung and the chest wall. Usually there should be between 10-20ml to lubricate the lung to aid coughing and deep breaths. Pleural effusion can build up for several reasons: cancer, congestive heart failure, pneumonia and pulmonary embolism and lots of other reasons. If a clear cause cannot be established, a pleural tap will be performed. This is where a small needle is placed inside the pleural space and the fluid is taken off with a syringe. The samples will then be sent off to many different places for testing. Including: Cytology, biochemistry and microbiology. The fluid will either be exudative or transudative. Exudative fluid occurs due to inflammation and increased capillary permeability, things like pneumonia, cancer, TB and viral infection. Transudative fluid occurs due to increased hydrostatic pressure which is caused by things like heart failure and cirrhosis. The tests that are sent off will differentiate between the two. Learning why the fluid is there will then lead to more treatment or surgery.