2.19 Pleural Effusions: Transudate vs Exudate

Pulmonary system review for the USMLE Step 1 Exam

• Pleural effusion is an abnormal accumulation of fluid in the pleural cavity.
• Pleural effusions can be characterized as either exudative or transudative based on the content of the extra fluid.
• Exudative effusions have high protein and lactate dehydrogenase content, while transudative effusions have low protein and lactate dehydrogenase content.
• Pleural exudates are commonly caused by lung infections, lung cancer, and inflammatory diseases.
• Pleural transudates are caused by increased hydrostatic pressures or decreased oncotic pressure within the lung capillaries that force fluid into the pleural cavity.
• Transudative effusions are not caused by inflammation, so lactate dehydrogenase content is not expected to be high.
• Proteins are too big to move between cells, so transudative effusions have low protein content.