What are the Light's criteria for diagnosing pleural effusion?

From the Guidelines

A pleural effusion is classified as an exudate if it meets at least one of the following Light's criteria: 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 for serum LDH, as stated in the most recent guideline 1. The criteria are used to differentiate between transudative and exudative pleural effusions, which is crucial for determining the underlying cause and guiding treatment.

• The pleural fluid protein to serum protein ratio greater than 0.5 indicates increased capillary permeability, suggesting an exudative process.
• The pleural fluid LDH to serum LDH ratio greater than 0.6 also points towards an exudative effusion, often associated with inflammatory or malignant conditions.
• A pleural fluid LDH greater than two-thirds the upper limit of normal for serum LDH is another indicator of an exudative effusion, which may result from various conditions, including infections or malignancies. These criteria have been consistently supported by recent studies, including the European Respiratory Journal 1 and the British Thoracic Society guideline 2, although the most recent and highest quality study is preferred 1. To apply Light's criteria, simultaneous collection and analysis of pleural fluid and serum samples for protein and LDH levels are necessary. It is essential to consider potential factors that may influence the results, such as diuretic therapy, which can sometimes cause a transudate to appear as an exudate, and calculating the serum-pleural fluid albumin gradient may provide additional clarification in such cases. The sensitivity and specificity of Light's criteria in identifying exudates and transudates are critical factors in clinical decision-making, with approximately 98% sensitivity for identifying exudates, as noted in previous studies, although the exact sensitivity and specificity may vary depending on the population and context.

From the Research

Light's Criteria for Diagnosing Pleural Effusion

The Light's criteria are used to determine whether a patient has a transudative or exudative pleural effusion by measuring the concentrations of protein and lactate dehydrogenase (LDH) in the pleural fluid and serum 3, 4, 5.

• The criteria involve comparing the pleural fluid protein and LDH levels to those in the serum.
• If the effusion is transudative, therapy should be directed toward the underlying congestive heart failure, cirrhosis, or nephrosis 3, 4.
• The Light criteria have been shown to exhibit high sensitivity (98%) in detecting exudative pleural effusions, but have a limited capacity to rule out transudates 6.
• The criteria may misclassify about 25% of transudates as exudates, particularly in patients on diuretics 7.

Application of Light's Criteria

To apply Light's criteria, the following measurements are taken:

• Pleural fluid protein to serum protein ratio
• Pleural fluid LDH to serum LDH ratio
• Pleural fluid LDH level 3, 5, 6
• If the patient's pleural fluid meets exudative criteria, but the patient appears clinically to have a transudative effusion, then the serum-pleural fluid albumin gradient should be measured 5, 7.

Limitations and Alternatives

While Light's criteria are useful, they may not always accurately categorize pleural effusion as transudate or exudate 6, 7.

• The serum-effusion albumin gradient (SEAG) method has been shown to be an effective alternative diagnostic tool for identifying transudates misclassified by Light's criteria 6.
• Pleural effusion glucose levels may also be useful in accurately categorizing pleural effusion as transudate or exudate 6.