How is a pleural effusion classified and what distinguishes a transudative effusion from an exudative effusion?

Pleural effusions are classified as transudative or exudative based on their protein content. Transudative effusions contain protein-poor, cell-poor fluid and are typically clear, while exudative effusions are rich in protein and occur due to inflammatory reactions.

Transudative effusions arise mainly from non-inflammatory conditions and are characterized by low protein content, often resulting from increased hydrostatic pressure or decreased oncotic pressure. In contrast, exudative effusions result from increased capillary permeability due to inflammation, leading to fluid that is rich in protein. Common causes of transudative effusions include heart failure and liver disease, whereas exudative effusions are often associated with infections or malignancies.