Pleural Fluid Proteins

Function

Pleural fluid protein measurement is used to evaluate the composition of fluid accumulated in the space between the lungs and the chest wall (pleural effusion). The protein concentration is a primary component of 'Light’s Criteria,' which is the gold standard for distinguishing between two types of effusions: transudates and exudates. This distinction is the most critical step in narrowing down the cause of lung-related fluid buildup.

Why it is Ordered

This test is performed on fluid obtained via thoracentesis. It is ordered to:

• Determine the Pathophysiology: To see if the fluid is leaking due to pressure imbalances (transudate) or local inflammation/injury (exudate).
• Guide Treatment: The treatment for heart failure-related fluid is vastly different from the treatment for pneumonia-related fluid.
• Diagnostic Narrowing: Helping to decide if further tests (like cytology or cultures) are necessary.

Associated Conditions

• Transudative Effusions (Low Protein): Usually caused by systemic factors like Congestive Heart Failure (CHF), Cirrhosis, or Nephrotic Syndrome. In these cases, the pleural protein to serum protein ratio is usually ≤ 0.5.
• Exudative Effusions (High Protein): Caused by local disease. Common causes include Pneumonia, Malignancy (lung or breast cancer), Pulmonary Embolism, or autoimmune diseases like Rheumatoid Arthritis. These typically have a fluid protein to serum protein ratio > 0.5. Identifying the protein level allows the physician to stop looking at systemic pressure issues and start looking for local infections or tumors.