What are the laboratory values that distinguish transudate from exudate in pleural (peritoneal) fluid analysis?

From the Guidelines

Pleural fluid is classified as an exudate if it meets one or more of the following criteria: pleural fluid protein divided by serum protein is > 0.5, pleural fluid lactate dehydrogenase (LDH) divided by serum LDH is > 0.6, or pleural fluid LDH > 2/3 the upper limits of laboratory normal value for serum LDH, as per the most recent guidelines 1. To distinguish between transudate and exudate, laboratory values are crucial. The key parameters include:

• Pleural fluid protein to serum protein ratio
• Pleural fluid LDH to serum LDH ratio
• Pleural fluid LDH levels in relation to the upper limit of normal serum LDH These criteria, known as Light's criteria, help in identifying the nature of the pleural effusion, which is vital for determining the underlying cause and guiding treatment decisions 2. In clinical practice, understanding the difference between transudates and exudates is essential because it reflects different pathophysiological processes. Transudates typically result from systemic conditions affecting fluid balance, such as heart failure or cirrhosis, whereas exudates are indicative of local inflammatory or disease processes, including infections or malignancies. The most recent and highest quality study 1 provides clear guidelines on the thresholds for classifying a pleural effusion as an exudate, emphasizing the importance of these laboratory values in clinical decision-making.

From the Research

Transudate versus Exudate Lab Values

• The distinction between transudates and exudates is crucial in the diagnosis of pleural effusions, with various parameters used to differentiate between the two 3.
• Lab values such as total protein, albumin, lactate dehydrogenase (LDH), cholesterol, and bilirubin are commonly measured in pleural fluid to aid in this distinction 3, 4.
• The pleural fluid to serum albumin ratio >0.5 has been found to be a highly accurate parameter for differentiating between exudates and transudates, with an accuracy of 88.4% 3.
• Light's criteria and abbreviated Light's criteria are also commonly used, with similar accuracies of 87.8% and 88.2%, respectively 3.
• Cholesterol and LDH measurements in fluids have been found to be sensitive and specific markers for differentiating between exudates and transudates, with a sensitivity of 90% and a specificity of 79% 4.
• The use of diuretics can affect the characteristics of pleural fluid, with increases in LDH, protein, and cholesterol levels, but it is uncommon for a transudative pleural effusion to develop the characteristics of an exudative pleural effusion with rapid diuresis 5, 6.

Effect of Diuretics on Pleural Fluid Characteristics

• Diuretic therapy can increase the total protein and LDH concentrations in pleural fluid in patients with transudates due to heart failure 5.
• The concentrations of biochemical components in pleural fluid, such as albumin, cholesterol, and cholinesterase, also increase progressively during diuretic therapy 5.
• The calculation of serum-pleural fluid gradients for protein and albumin may be useful in distinguishing transudates from exudates in patients with congestive heart failure who have undergone diuresis 5.
• Aggressive diuresis can result in significant increases in LDH and protein levels in pleural fluid, but it is rare for a transudative pleural effusion to become an exudate 6.