What are the criteria for diagnosing exudative pleural (lung) fluid?

From the Guidelines

Exudative pleural fluid is diagnosed using Light's criteria, which requires that the fluid meets at least one of the following conditions: pleural fluid protein to serum protein ratio greater than 0.5, pleural fluid LDH to serum LDH ratio greater than 0.6, or pleural fluid LDH greater than two-thirds the upper limit of normal serum LDH. The diagnosis of exudative pleural fluid is crucial in guiding further investigation into specific causes such as infection, malignancy, or inflammatory conditions.

Key Criteria

• Pleural fluid protein to serum protein ratio greater than 0.5
• Pleural fluid LDH to serum LDH ratio greater than 0.6
• Pleural fluid LDH greater than two-thirds the upper limit of normal serum LDH These criteria help differentiate exudates (caused by inflammation or malignancy) from transudates (caused by imbalances in hydrostatic or oncotic pressures) 1.

Additional Parameters

Additional parameters that can suggest an exudate include pleural fluid cholesterol greater than 55 mg/dL, or pleural fluid lactate dehydrogenase (LDH) greater than 67% the upper limit of normal, which can be used if a serum sample is not available 1.

Importance of Simultaneous Sampling

When evaluating pleural fluid, it's essential to collect both pleural fluid and serum samples simultaneously for accurate comparison 1.

Clinical Implications

The biochemical parameters used in Light's criteria reflect increased capillary permeability or lymphatic obstruction that allows protein-rich fluid to accumulate in the pleural space. The diagnosis of exudative pleural fluid guides further investigation and management, as exudates generally require more extensive workup than transudates 1.

From the Research

Exudative Pleural Fluid Criteria

The criteria for diagnosing exudative pleural fluid are based on several studies, including those by 2, 3, 4, 5, and 6. The key criteria include:

• Light's criteria, which include a ratio of pleural fluid protein to serum protein >0.5, a ratio of pleural fluid LDH to serum LDH >0.6, or pleural fluid LDH >two-thirds the upper limit of normal for serum LDH 2, 3, 6
• Pleural fluid cholesterol level >55 mg/dL 6
• Pleural fluid lactate dehydrogenase (LDH) level >200 U/L 6
• Ratio of pleural fluid cholesterol to serum cholesterol >0.3 6
• Pleural fluid C-reactive protein (CRP) levels, which can help differentiate between parapneumonic and tuberculous effusions 5

Diagnostic Tests

Several diagnostic tests can be used to evaluate exudative pleural effusions, including:

• Differential white cell count of the pleural fluid 3
• Cytology of the pleural fluid 3
• Levels of adenosine deaminase, glucose, amylase, and lactate dehydrogenase in the pleural fluid 3
• Pleural fluid CRP levels 5
• Helical computed tomography and thoracoscopy 2

Limitations and Complications

The diagnosis of exudative pleural effusions can be complicated by several factors, including:

• Transudates misidentified as exudates using Light's criteria 2, 3
• Pneumothorax, which can occur in up to 6.0% of thoracentesis procedures 6
• Chest tube placement, which may be required in up to 2.0% of procedures 6