Professional and Linguistic Assessments Board (PLAB) Practice Exam

If a pleural effusion's pleural protein to serum protein ratio is greater than 0.5, what does it indicate?

1. It is likely transudative.

2. It is likely exudative.

3. It is inconclusive.

4. It requires further testing.

The correct answer is: It is likely exudative.

A pleural effusion with a pleural protein to serum protein ratio greater than 0.5 indicates that it is likely exudative. This is based on the criteria established to differentiate between transudative and exudative pleural effusions, specifically Light's criteria. Exudative pleural effusions occur due to diseases affecting the pleura, such as infections, malignancies, or inflammatory diseases, which lead to increased permeability of the pleural membranes, resulting in a higher concentration of proteins in the pleural fluid compared to serum. In contrast, transudative effusions are typically caused by systemic conditions that increase hydrostatic or decrease oncotic pressure, leading to lower protein levels in the pleural fluid relative to serum. Therefore, a ratio above 0.5 suggests that the pleural effusion is due to a localized process rather than a systemic issue, making it indicative of an exudative process.