Pleural effusion is excess fluid accumulation between the outer surface of lungs and the chest wall.  A procedure called thoracocentesis is done to obtain the fluid sample. Causes of effusion include viral or bacterial infections, cancer or a general fluid balance disorder (kidney failure/ heart failure/ liver cirrhosis ).  Analysis of this fluid helps in identifying and classifying the type of fluid to assist with the diagnosis. High levels of an enzyme- adenosine deaminase in pleural fluid is suggestive of tuberculosis.