Exudate

Overview
An exudate is any fluid that filters from the circulatory system into lesions or areas of inflammation. Its composition varies but generally includes water and the dissolved solutes of the blood, some or all plasma proteins, white blood cells, platelets and (in the case of local vascular damage) red blood cells.

Pus is an example of exudate found in infected wounds that also includes bacteria and high concentrations of white blood cells. Clear blister fluid is an example of an exudate that contains water (and solutes) together with some plasma proteins, but not many blood cells.

Serous exudate is usually seen in mild inflammation, with little protein content. Its consistency resembles that of serum, and can usually be seen in certain disease states like tuberculosis.

Purulent or suppurative exudate consists of plasma with both active and dead neutrophils, fibrinogen, and necrotic parenchymal cells. This kind of exudate is consistent with more severe infections, and is commonly referred to as pus.

Fibrinous exudate is composed mainly of fibrinogen and fibrin. It is characteristic of rheumatic carditis, but is seen in all severe injuries such as strep throat and bacterial pneumonia. Fibrinous inflammation is often difficult to resolve due to the fact that blood vessels grow into the exudate and fill the space that was occupied by fibrin. Often, large amounts of antiboiotics are necessary for resolution.

Hemorrhagic exudate is seen in injury that causes rupture of blood vessels.

Pleural exudate has a specific gravity greater than 1.020. Certain conditions produce exudative pleural effusions such as infections, pulmonary infarctions, rheumatoid arthritis, lupus erythematosus, and certain malignancies.

Catarrhal exudate is seen in the nose and throat and is characterized by a high content of mucus.

There is an important distinction between transudates and exudates. Transudates are caused by disturbances of hydrostatic or colloid osmotic pressure, not by inflammation. Medical distinction between transudates and exudates is through the measurement of the specific gravity of extracted fluid. Specific gravity is used to measure the protein content of the fluid. The higher the specific gravity, the greater the likelihood of capillary permeability changes in relation to body cavities. For example, the specific gravity of the transudate is usually less than 1.012.