The objective: to evaluate diagnostic efficacy of adenosine deaminase and other biochemical markers (total protein, glucose, and lactate dehydrogenase) in pleural fluid in pleurisy of various etiologies. Subjects and Methods. The following parameters in pleural fluid and clinical data of 89 patients with pleural effusion of various etiologies were retrospectively analyzed: tuberculous (TBP), metastatic (MP) and other (OP) pleurisy. In the pleural fluid, the activity of ADA, lactate dehydrogenase (LDG), levels of total protein (TP), and glucose (Glu) were assessed. Results. ADA activity was statistically significantly higher in patients with tuberculous pleurisy versus patients with metastatic pleurisy and pleurisy of some other etiology. High enzyme activity was due to a statistically significant increase in activity of the ADA-2 isoenzyme. The LDG/ADA and LDG/ADA-2 ratios, the Glu level in patients with tuberculous plerisy were statistically significantly lower, and, on the contrary, the ADA/TP and ADA-2/TP, ADA/Glu and ADA-2/Glu ratios were higher than in patients from the other two groups. High diagnostic efficacy of ADA, ADA-2, LDG/ADA and LDG/ADA-2 was revealed: sensitivity of the tests made 91, 82, 100%, specificity made 87, 81, 83 and 92%, respectively. Conclusions. For differential diagnosis of tuberculous pleurisy, the LDG/ADA ratio can be used along with ADA which is the effective biomarker. The ratios of ADA/TP and ADA-2/TP, ADA/Glu and ADA-2/Glu activity are of little information value for the differential diagnosis of tuberculous pleurisy. © 2025 Elsevier B.V., All rights reserved.