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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 /microlit) 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|
|Main causes||↑ hydrostatic
|Specific gravity||< 1.012||> 1.020|
|Protein content||< 2.5 g/dL||> 2.9 g/dL|
|< 0.5||> 0.5|
Serum [albumin] - Effusion [albumin]
|> 1.2 g/dL||< 1.2 g/dL|
upper limit for serum
|< 0.6 or < 2⁄3||> 0.6 or > 2⁄3|
|Cholesterol content||< 45 mg/dL||> 45|
|Radiodensity on CT scan||2 to 15 HU||4 to 33 HU|
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.
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.
The most common causes of pathologic transudate include conditions that :
Exudate – extravascular fluid due to vessel alteration during inflammation (increased permeability, vascular constriction then dilation). This results in an extracellular fluid of high protein content, with cell debris present and high specific gravity (>1.020).
This is in contrast to transudate where the extracellular fluid is an ultrafiltrate of blood plasma and thus larger molecules such as proteins and cell debris are absent.