The closed cavities of the body—namely, the pleural, pericardial, and peritoneal cavities—are each lined by two membranes, visceral membrane and parietal membrane.

There is a small amount of fluid between the membranes that is an ultrafiltrate of plasma. When the production and reabsorption of the ultrafiltrate is not balanced, fluid may accumulate, resulting in effusion. Effusions may be classified as transudate or exudate.


Classifying a serous fluid as transudate or exudate can provide a valuable initial diagnostic step and aid diagnosis of underlying cause. Followings are the differences between transudate and exudate :

Differences Between Transudates and Exudates Table:

CauseUsually develop from imbalances in hydrostatic and oncotic forces in circulation.Usually develop from increased capillary permeability or decreased lymphatic reabsorption.
Associated withCongestive heart failure Fluid overload Nephrotic syndrome Hepatic cirrhosis MalnutritionMicrobial infections Membrane inflammations Malignancy Connective tissue diseases.
AppearanceClear, thin-colored, pale yellowTurbid, hemorrhagic, straw colored
FibrinogenLow content of fibrinogen (low tendency to clot)High content of fibrinogen (high tendency to clot)
Specific gravity<1.012>1.012
Glucose contentSame as plasmaLow (less than 60 mg/dl)
Total ProteinLess than 3 gm/dlMore than 3 gm/dl
Fluid/Serum Protein<0.5>0.5
LDH<0.67 x UNL Serum>0.67 x UNL Serum
WBC count<1000/ul>1000/ul
Differential countMesothelial cells or lymphocytesPolymorphs, lymphocytes or RBCs

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