Pleural Effusion: Transudate vs. Exudate

Pleural membrane is the layer of membrane the covers lung from the pleural space.
Pleural space is the space between the lung and the chest wall; it’s a space where the lung can expand to when the person inhales.
Effusion is the escape of fluid from the normal vessels by rupture or abnormal transition.
When there is a pathology involving the lung such as from pneumonia (lung infection) or cancer, fluid from the vessels surrounding the lung tend to exit out in the interstitial space and in the pleural space, hence effusion. Effusion can be classified in to two types: the lighter effusion is called transudate and the heavier effusion is called exudate.
Pleural Effusion
Transudate effusion is due to an imbalance between hydrostatic and oncotic pressures that increases fluid movement across the capillaries into the visceral pleura and the pleural space. Transudates fluid does not require further intervention except for treatment for the underlying cause.
Exudative effusions are due to capillary membrane permeability caused by pleural and lung inflammation. A specific criteria called the Light criteria, defines exudate:
  • Pleural fluid protein/serum protein ratio >0.5
  • Pleural fluid lactate dehydrogenase (LDH)/serum LDH ratio >0.6
  • Pleural fluid LDH >2/3 of the upper limit of normal for serum LDH
Exudate effusion will also have a criteria of pleural fluid glucose <60 mg/dL due to the high metabolic rate of leukocytes (and/or bacteria) within the fluid.  

Updated 6/14/2014
The lungs are not the only areas of effusion; other areas can have effusion as well such as the peritoneum (the abdominal area).
Causes of Transudate Effusion

– Congestive Heart failure
– Cirrhosis (portal hypertension and hypoalbuminemia)
– Peritoneal dialysis

Causes of Exudate Effusion
– Pneumonia
– Malignancy (usually lung cancer in men and breast cancer in women); when there is a large unilateral pleural effusions then it’s mostly due to malignancy

Image source: Clevelend Clinic