Transudate

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 /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 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]

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.

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 :

• Increase hydrostatic pressure in vessels : left ventricular heart failure,
• Decrease colloid oncotic pressure in blood vessels :
  • Cirrhosis (Cirrhosis leads to hypoalbuminemia and decreasing of colloid oncotic pressure in plasma that causes edema)
  • Nephrotic syndrome (also due to hypoalbuminemia caused by proteinuria).
  • Malnutrition (hypoalbuminism)

See also

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.

References

1. The University of Utah • Spencer S. Eccles Health Sciences Library > WebPath images > "Inflammation".
2. Heffner J, Brown L, Barbieri C (1997). "Diagnostic value of tests that discriminate between exudative and transudative pleural effusions. Primary Study Investigators". Chest. 111 (4): 970–80. doi:10.1378/chest.111.4.970. PMID 9106577.
3. Light R, Macgregor M, Luchsinger P, Ball W (1972). "Pleural effusions: the diagnostic separation of transudates and exudates". Ann Intern Med. 77 (4): 507–13. doi:10.7326/0003-4819-77-4-507. PMID 4642731.
4. Roth BJ, O'Meara TF, Gragun WH (1990). "The serum-effusion albumin gradient in the evaluation of pleural effusions". Chest. 98 (3): 546–9. doi:10.1378/chest.98.3.546. PMID 2152757.
5. Cullu, Nesat; Kalemci, Serdar; Karakas, Omer; Eser, Irfan; Yalcin, Funda; Boyaci, Fatma Nurefsan; Karakas, Ekrem (2013). "Efficacy of CT in diagnosis of transudates and exudates in patients with pleural effusion". Diagnostic and Interventional Radiology. 20: 116–20. doi:10.5152/dir.2013.13066. ISSN 1305-3825. PMC 4463296. PMID 24100060.
6. The University of Utah • Spencer S. Eccles Health Sciences Library; WebPath images "Inflammation".
7. "IM Quiz: Pleural Adenocarcinoma". Archived from the original on 2008-09-16.