Which pleural fluid parameter best indicates exudate rather than transudate?

• A. Pleural fluid to serum protein ratio greater than 0.5
• B. Color
• C. Leukocyte count
• D. Glucose level

Answer: (A)

Distinguishing exudates from transudates hinges on how much protein leaks into the pleural space due to inflammation or malignancy. When the pleural fluid protein level is high relative to the serum, it signals increased vascular permeability and local pathology, which is characteristic of an exudate. The pleural fluid to serum protein ratio, with a cutoff greater than 0.5, directly reflects this leakage pattern and is a classic single measure used to identify exudates. This aligns with broader criteria for pleural effusions, which emphasize protein and LDH changes, but the protein ratio is the most straightforward and reliable single discriminator.

Color is too nonspecific to distinguish exudates from transudates, as it can vary with many factors and doesn’t reveal the underlying process. Leukocyte count can be elevated in both types of effusions and therefore doesn’t reliably differentiate them. Glucose level can be affected by infection or inflammatory conditions but is not a definitive standalone indicator of exudate versus transudate.