Which of the following pleural fluid parameters is most specific for diagnosing an exudative pleural effusion according to Light's criteria?

Explanation

## Light's Criteria for Exudative Effusion Light's criteria define an exudative pleural effusion when **at least one** of the following is present: | Criterion | Threshold | |-----------|----------| | **Pleural/serum protein ratio** | > 0.5 | | **Pleural/serum LDH ratio** | > 0.6 | | **Pleural LDH** | > 2/3 of upper limit of normal serum LDH | **Key Point:** The LDH criterion is the most commonly used and most specific parameter because LDH rises in inflammatory/malignant conditions and is less affected by nutritional or renal factors. **High-Yield:** Light's criteria have ~95% sensitivity and ~95% specificity for identifying exudates. If all three criteria are negative, the effusion is a transudate with near-certainty. **Clinical Pearl:** When serum and pleural fluid samples are obtained simultaneously, the protein and LDH ratios are preferred over absolute values because they account for individual variation in serum levels. **Mnemonic:** **LPP** = Light's Pleural Parameters (LDH ratio, Protein ratio, and absolute Pleural LDH).