The role of pleural fluid lactate dehydrogenase-to-adenosine deaminase ratio in differentiating the etiology of pleural effusions.

Exudative pleural effusion includes tuberculous pleural effusion (TPE), parapneumonic pleural effusion (PPE), and malignant pleural effusion (MPE). An elevated pleural fluid adenosine deaminase (ADA) typically implies TPE, but the rule may not apply to every individual case. Recent studies proposed that the pleural fluid lactate dehydrogenase (LDH)-to-ADA ratio showed a higher diagnostic power than pleural fluid ADA alone in differentiating the etiology of pleural effusion. Hence, we aimed to investigate the performance of pleural fluid LDH-to-ADA ratio as a biomarker in assistance with the diagnosis of TPE, PPE, and MPE. All patients who underwent thoracentesis for the first time with a pleural fluid ADA >40 U/L were included in this retrospective study. The clinical data including pleural fluid ADA and LDH-to-ADA ratio were analyzed. A total of 311 patients were enrolled during the study interval. The pleural fluid LDH-to-ADA ratio <14.2 (sensitivity: 74.2%; specificity: 90.4%) favored TPE, while the pleural fluid LDH-to-ADA ratio >14.5 (sensitivity: 79.9%; specificity: 78.5%) favored PPE. Besides, the pleural fluid LDH-to-ADA ratio >46.7 (sensitivity: 56.3%; specificity: 78.3%) favored MPE owing to primary lung cancers. In conclusion, the pleural fluid LDH-to-ADA ratio was an effective indicator in differentiating the etiology of pleural effusions in the cases of high ADA level in the pleural fluid.