Lights Criteria for Pleural Effusions

Overview

• Lights Criteria is a clinical tool used to determine whether a pleural effusion is exudative.
• Understanding pleural effusions is crucial for exams like USMLE, COMLEX, and shelf exams.

Types of Pleural Effusions

Exudative Pleural Effusion

• Characterized by: Extra substances in the pleural space.
• Caused by: Increased capillary permeability.
• Examples of disease states include:
  • Inflammation
  • Pneumonia
  • Cancer
  • Tuberculosis (TB)
  • Pulmonary emboli

Transudative Pleural Effusion

• Characterized by: Fluid overload.
• Caused by: Increased capillary pressure or decreased oncotic pressure.
• Examples of disease states include:
  • Heart failure
  • Cirrhosis
  • Nephrotic syndrome

Key Concept

• Exudative = Extra (extra stuff in pleural effusion)
• Transudative = Fluid overload

Lights Criteria: Identifying Exudative Effusions

• There are three criteria:

  1. Pleural fluid protein/serum protein ratio > 0.5
     • Indicates more protein in pleural fluid compared to serum due to cell breakdown.
  2. Pleural fluid LDH/serum LDH ratio > 0.6
     • Represents higher LDH in pleural fluid due to enzyme leakage from cell breakage.
  3. Pleural fluid LDH > 2/3 the upper limit of normal for serum LDH
     • Indicates significant enzyme leakage.

• Rule of Thumb: If any of these three criteria are true, the effusion is exudative.

Understanding the Mechanisms

• Inflammation & Cancer:
  • Cause cell breakage, releasing protein and LDH into the pleural space.
  • This results in higher concentrations of these substances in the pleura compared to the serum.

Clinical Application

• On exams like USMLE or COMLEX:
  • Look for high pleural protein and LDH as signs of exudative effusions.
  • The distinction between exudative and transudative effusions is frequently tested.

Conclusion

• Lights Criteria is essential for diagnosing the type of pleural effusion.
• Remembering the simple association of exudative with extra substances and transudative with fluid overload can help in clinical decision-making and exam settings.