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 | ||
|---|---|---|
| Transudate | Exudate | |
| Main causes | ↑ hydrostatic pressure, ↓ colloid osmotic pressure |
Inflammation-Increased vascular permeability |
| Appearance | Clear[1] | Cloudy[1] |
| Specific gravity | < 1.012 | > 1.020 |
| Protein content | < 2.5 g/dL | > 2.9 g/dL[2] |
| fluid protein/ serum protein |
< 0.5 | > 0.5[3] |
| SAAG = Serum [albumin] - Effusion [albumin] |
> 1.2 g/dL | < 1.2 g/dL[4] |
| fluid LDH upper limit for serum |
< 0.6 or < 2⁄3 | > 0.6[2] or > 2⁄3[3] |
| Cholesterol content | < 45 mg/dL | > 45 |
| Radiodensity on CT scan | 2 to 15 HU[5] | 4 to 33 HU[5] |
|
See also: Light's criteria for pleural effusions |
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.[6]
Levels of lactate dehydrogenase (LDH)[7] or a Rivalta test can be used to distinguish transudate from exudate.[citation needed]
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.[citation needed]
The most common causes of pathologic transudate include conditions that:[citation needed]