Transudate – fluid created as a result of changes in hydrostatic pressure. E.g. Cirrhosis, Heart Failure, Nephrotic Syndrome, Superior Vena Cava Obstruction. Here, only fluid moves from the intravascular space into the extravascular space. There is no inflammation of the vessels and so proteins don’t move out.
Exudate – fluid created as a result of an inflammatory process. The walls of the vessels are inflamed, the spaces between endothelial cells are increased, and both proteins and fluids leak into the surrounding tissue to form fluid there. E.g. Pulmonary Embolus, Pneumonia, etc.
The protein and lactate dehydrogenase (LDH) levels in pleural fluid can help differentiate between transudative and exudative effusions.
Light’s criteria (= Exudate Criteria) are 99.5% sensitive for diagnosing exudative effusions and differentiate exudate from transudative effusions in 93-96% of cases.
Light’s criteria for fluid being exudate are:
- Pleural fluid protein to serum protein ratio > 0.5,
- Pleural fluid LDH to serum LDH ratio > 0.6, and/or
- pleural LDH > 2/3rds (i.e. 0.67 times) the upper limit of normal for serum LDH)
NB: 0.67 = 2/3
Q: What is the source of LDH in pleural fluid?
https://www.ncbi.nlm.nih.gov/pubmed/3749700