From the Guidelines
To diagnose exudative fluid using Light's criteria, the fluid must meet at least one of the following conditions:
- The ratio of fluid protein to serum protein is greater than 0.5
- The ratio of fluid LDH to serum LDH is greater than 0.6
- The fluid LDH is greater than two-thirds the upper limit of the normal serum LDH value, as outlined in the British Thoracic Society guideline for pleural disease 1 and supported by the ERS statement on benign pleural effusions in adults 1. If any of these conditions are met, the fluid is considered exudative. Light's criteria have a high sensitivity of 98% for identifying exudates, but a lower specificity of 72%, as noted in the ERS statement 1. These criteria are widely used in clinical practice due to their ability to maximize the detection of exudates and reduce the risk of missing potentially serious conditions, such as malignancy or infection.
From the Research
Light's Criteria for Exudative Pleural Fluid
Light's criteria are used to identify exudative pleural fluid and distinguish it from transudative fluid. The criteria include:
- Pleural fluid/serum total protein ratio greater than 0.5
- Pleural fluid/serum LDH ratio greater than 0.6
- Pleural fluid LDH greater than 200 U/L or greater than two-thirds of the upper normal limit in serum 2, 3
Rationale Behind the Criteria
These criteria were established to help differentiate between exudative and transudative pleural effusions, with exudative effusions typically having higher protein and LDH levels due to increased permeability of the pleural membranes 4, 5
Limitations and Potential Misclassifications
While Light's criteria are sensitive for identifying exudative pleural effusions, they can sometimes misclassify transudative effusions, particularly those caused by congestive heart failure, as exudates 2, 6