Transudate is extravascular fluid with low protein content and a low specific gravity (< 1.012). It has low nucleated cell counts (less than 500 to 1000 per microliter) and the primary cell types are mononuclear cells: macrophages, lymphocytes and mesothelial cells. For instance, an ultrafiltrate of blood plasma is transudate. It results from increased fluid pressures or diminished colloid oncotic forces in the plasma.

Transudate vs. exudate

There is an important distinction between transudates and exudates. Transudates are caused by disturbances of hydrostatic or colloid osmotic pressure, not by inflammation. They have a low protein content in comparison to exudates and thus appear clearer. Levels of lactate dehydrogenase (LDH) or a Rivalta test can be used to distinguish transudate from exudate. Their main role in nature is to protect elements of the skin and other subcutaneous substances against the contact effects of external climate and the environment and other substances – it also plays a role in integumental hygiene.

Pathology

The most common causes of pathologic transudate include conditions that: