Pleural Fluid - Biochemical Analysis

Function

Biochemical analysis of pleural fluid is a comprehensive battery of tests performed on fluid aspirated from the pleural cavity. The primary goal is to apply Light’s Criteria to distinguish between a 'transudate' and an 'exudate'. Transudates are caused by systemic factors (pressure changes), while exudates are caused by local lung or pleural disease (inflammation or capillary leak).

Why it is Ordered

When a patient has a pleural effusion, clinicians must determine the cause to provide treatment. Analysis typically includes:

• Total Protein and Albumin: To compare with serum levels.
• Lactate Dehydrogenase (LDH): A marker of inflammation and cell turnover.
• Glucose: Low levels can indicate infection (bacteria consuming glucose) or malignancy.
• pH: Acidic pleural fluid (pH < 7.2) often indicates a complicated infection requiring drainage.

Associated Conditions

• Congestive Heart Failure (CHF): Most common cause of transudates.
• Pneumonia: Leads to parapneumonic effusions (exudate).
• Malignancy: Lung or breast cancer spreading to the pleura.
• Pulmonary Embolism: Can cause either transudates or exudates.

Interpretation

According to Light's Criteria, an effusion is an exudate if: 1) Pleural fluid protein/serum protein ratio > 0.5; 2) Pleural fluid LDH/serum LDH ratio > 0.6; or 3) Pleural fluid LDH > 2/3 the upper limit of normal for serum LDH. Exudates require extensive workup for infection or cancer, while transudates usually respond to treating the underlying systemic condition (like diuretics for heart failure).