According to Light's criteria, what indicates a transudative pleural effusion?

• A. Low lactate dehydrogenase
• B. High protein ratio
• C. Low pleural glucose
• D. Presence of lymphocytes

Answer: (A)

Light's criteria are used to distinguish between transudative and exudative pleural effusions based on the composition of pleural fluid. According to these criteria, a pleural effusion is considered transudative if it meets at least one of the following criteria: the pleural fluid protein to serum protein ratio is less than 0.5, the pleural fluid lactate dehydrogenase (LDH) to serum LDH ratio is less than 0.6, and the pleural fluid LDH is less than two-thirds the upper limit of normal for serum LDH.

In this case, the indication of a transudative pleural effusion with low lactate dehydrogenase aligns with the second criterion in Light's criteria, suggesting that the effusion is likely caused by systemic factors such as heart failure, cirrhosis, or nephrotic syndrome rather than local inflammatory processes.

The other choices present features more typical of exudative pleural effusions. A high protein ratio is indicative of an exudative process, as is a low pleural glucose level due to metabolic activity in malignancies or infections. The presence of lymphocytes can often be associated with tuberculosis or malignancy, which also