The presence of serum protein in pleural fluid indicates what?

• A. Transudate
• B. Exudate
• C. Empyema
• D. Chylothorax

Answer: (B)

When pleural fluid contains a noticeable amount of protein, it points to an exudative process. This happens because inflammation or infection in the pleura increases capillary permeability, allowing larger molecules like proteins from the serum to leak into the pleural space. In contrast, transudates arise from systemic imbalances (like high hydrostatic pressure or low oncotic pressure) that cause fluid to seep out without large amounts of protein, so the protein content is comparatively low.

In clinical practice this distinction is formalized by Light’s criteria, where a higher protein concentration in pleural fluid relative to serum supports an exudate. Exudates cover conditions such as pneumonia, TB, malignancy, and pancreatitis, and among them empyema is an infected exudate, while chylothorax is a lipid-rich exudate from lymphatic leakage. So the presence of serum protein in the pleural fluid best indicates an exudative effusion.