American Board of Internal Medicine (ABIM) Certification Practice Exam

What ratio of pleural fluid LDH to serum LDH indicates exudative pleural effusion according to Light's Criteria?

1. Less than 0.4

2. Equal to 0.5

3. Greater than 0.6

4. Greater than 1.0

The correct answer is: Greater than 0.6

Light's Criteria are pivotal in distinguishing between transudative and exudative pleural effusions based on the analysis of pleural fluid and serum. One of the key components of these criteria involves the ratio of pleural fluid lactate dehydrogenase (LDH) to serum LDH. When the ratio of pleural fluid LDH to serum LDH exceeds 0.6, it indicates that the pleural effusion is exudative. Exudative effusions are typically associated with conditions such as infections (like pneumonia), malignancy, and inflammatory diseases, where there is an increase in capillary permeability or pleural surface inflammation. This permeability allows proteins and LDH to spill into the pleural space, leading to a higher pleural fluid LDH level relative to serum. In contrast, a ratio lower than 0.6 generally suggests a transudative process, where the effusion is typically due to systemic conditions like heart failure or cirrhosis, characterized by normal capillary permeability. Therefore, the correct answer indicating an exudative process is the situation where the pleural fluid LDH to serum LDH ratio is greater than 0.6. This criterion helps clinicians quickly assess the underlying cause of ple