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Pleural fluid is found in the pleural cavity and serves as a lubricant for the movement of the lungs during inhalation and exhalation. It is derived from a plasma filtrate from blood capillaries and is found in small quantities between the layers of the pleurae - membranes that cover the chest cavity and the outside of each lung.

A variety of conditions and diseases can cause inflammation of the pleurae (pleuritis) and/or excessive accumulation of pleural fluid (pleural effusion). Pleural fluid analysis comprises a group of tests used to determine the cause. There are two main reasons fluid may collect in the pleural space:

1.Fluid may accumulate in the pleural space because of an imbalance between the pressure within blood vessels—which drives fluid out of blood vessels—and the amount of protein in blood—which keeps fluid in blood vessels. The fluid that accumulates in this case is called a transudate. This type of fluid usually involves both lungs and is often a result of either cirrhosis or congestive heart failure.

2.Fluid accumulation may be caused by injury or inflammation of the pleurae, in which case the fluid is called an exudate. It usually involves one lung and may be seen in infections (pneumonia, tuberculosis, sarcoidosis), malignancies (lung cancer, metastatic cancer, lymphoma, mesothelioma), rheumatoid disease, or systemic lupus erythematosus.

Differentiation between the types of fluid is important because it helps diagnose the specific disease or condition. Doctors and laboratory scientists use an initial set of tests (cell count, albumin and appearance of the fluid) to distinguish between transudates and exudates. Once the fluid is determined to be one or the other, additional tests may be performed to further pinpoint the disease or condition causing pleuritis and/or pleural effusion.