What criteria are utilized to differentiate between transudative and exudative pleural effusions?

The differentiation between transudative and exudative pleural effusions is primarily based on Light’s criteria. These criteria assess the fluid's protein levels and lactate dehydrogenase (LDH) levels in comparison to those in serum.

Specifically, Light's criteria state that an effusion is considered exudative if it meets any of the following three conditions:

1. The pleural fluid protein level is greater than 0.5 times the serum protein level.

2. The pleural fluid LDH level is greater than 0.6 times the serum LDH level.

3. The pleural fluid LDH level exceeds two-thirds of the normal upper limit for serum LDH.

If the effusion does not meet any of these criteria, it is classified as transudative. Transudative effusions typically result from systemic issues like heart failure or cirrhosis, while exudative effusions are often related to localized disease processes, such as infections or malignancy. Understanding these criteria helps clinicians accurately diagnose the underlying cause of the pleural effusion and guide appropriate management strategies.