Lactate dehydrogenase / adenosine deaminase (LDH/ ADA) ratio in pleural fluid for the diagnosis of infectious pleurisy

Background: Infectious pleurisy, including Tuberculous Pleural Effusion (TPE) and Para Pneumonic Effusion (PPE), remains a diagnostic challenge in resource-limited settings. Biomarkers such as Adenosine Deaminase (ADA) and Lactate Dehydrogenase (LDH) are widely used, but the diagnostic performance of their ratio (LDH/ADA) is less explored. To determine the diagnostic accuracy of pleural fluid LDH/ADA ratio in identifying infectious pleurisy, using pleural fluid culture as the gold standard. Material and Methods: This descriptive study was conducted in the Department of Pulmonology, Ojha Institute of Chest Diseases, Dow University of Health Sciences (DUHS), Karachi, from February 2025 till July 2025. In this descriptive study, 80 patients aged 18–60 years with pleural effusion and ADA > 33 U/L were recruited at Ojha Institute of Chest Diseases, DUHS, Karachi. Pleural fluid samples were analyzed for biochemical markers and culture. LDH/ADA ratio > 10 was considered positive. Diagnostic accuracy parameters were calculated. Results: A total of 80 patients with pleural effusion were included (mean age 37.8 ± 11.3 years; 60% male). Infectious effusions showed significantly higher pleural LDH, ADA, LDH/ADA ratio, and protein, whereas glucose and pH were lower compared to non-infectious effusions (p < 0.05). An LDH/ADA ratio > 10 demonstrated excellent diagnostic performance with 93.3% sensitivity, 90.0% specificity, and 92.5% overall accuracy. Conclusion: The pleural fluid LDH/ADA ratio demonstrates high diagnostic accuracy for infectious pleurisy and may serve as a reliable, inexpensive tool in clinical settings where pleural biopsy is unavailable. Keywords: Infectious pleurisy, LDH/ADA ratio, Pleural effusion, Para pneumonic effusion, Tuberculous pleural effusion.