In the context of pleural effusion, what does a lower LDH ratio indicate?

A lower lactate dehydrogenase (LDH) ratio in the context of a pleural effusion suggests a transudative effusion. Transudative effusions are generally characterized by lower protein and LDH levels compared to serum, which occurs due to systemic factors affecting fluid balance, such as congestive heart failure, cirrhosis, or nephrotic syndrome. The ratio of pleural fluid LDH to serum LDH is a crucial factor in differentiating transudative from exudative effusions. When this ratio is less than 0.6, it indicates that the effusion is more likely transudative.

In contrast, exudative effusions, which often occur due to localized processes like infection, malignancy, or inflammatory conditions, typically exhibit higher LDH levels and a corresponding increased LDH ratio. Therefore, understanding the significance of the LDH ratio is essential in diagnosing the underlying cause of a pleural effusion accurately.