Which statement accurately reflects Light's criteria for exudate in pleural fluid?

• A. All three thresholds must be met.
• B. Only the pleural/serum protein ratio matters.
• C. Any one of the thresholds indicates exudate.
• D. Pleural LDH alone determines exudate.

Answer: (C)

Light's criteria distinguish exudates from transudates by three parallel thresholds. If any one of these is met, the pleural fluid is classified as an exudate: pleural fluid protein divided by serum protein greater than 0.5; pleural fluid LDH divided by serum LDH greater than 0.6; or pleural fluid LDH more than two-thirds the upper limit of normal for serum LDH. The important point is that meeting at least one criterion is enough to label the fluid an exudate, reflecting the inflammatory or neoplastic processes that increase vascular permeability. If none of these thresholds are met, the fluid is considered a transudate. So the statement that any one of the thresholds indicates exudate correctly reflects Light's criteria.