Diagnostic Utility of Pleural Fluid Lactate and Pleural Fluid-to-Serum Lactate Ratio in Differentiating Various Etiologies of Exudative Pleural Effusions: A Cross-Sectional Observational Study

Background: Exudative pleural effusion presents a frequent diagnostic dilemma in clinical practice, particularly in regions where tuberculosis is highly prevalent. Although Light’s criteria, adenosine deaminase (ADA), and cytological analysis are routinely used, their diagnostic performance is limited by issues related to specificity, sensitivity, and processing time. Recently, pleural fluid lactate and the pleural fluid-to-serum (P/S) lactate ratio have been investigated as adjunctive biochemical markers for etiological differentiation. Objectives: The primary objectives of this study were to evaluate pleural fluid lactate levels and to assess the diagnostic performance of the P/S lactate ratio in patients with exudative pleural effusion. Additionally, the study aimed to determine the utility of these biomarkers in differentiating tubercular, malignant, parapneumonic, and other etiologies using receiver operating characteristic (ROC) curve analysis. Methods: This cross-sectional observational study included 50 adult patients diagnosed with exudative pleural effusion according to Light’s criteria. Based on clinical, radiological, cytological, and microbiological findings, patients were categorized into tubercular, malignant, parapneumonic, and miscellaneous groups. Pleural fluid and serum lactate levels were measured using a point-of-care blood gas analyzer, and the P/S lactate ratio was calculated. Diagnostic performance was analyzed using ROC curves. Results: Tubercular effusion was identified in 27 patients (54%), malignant effusion in 13 (26%), parapneumonic effusion in seven (14%), and miscellaneous causes in three (6%). Median pleural fluid lactate levels were highest in parapneumonic effusions (7.6 mmol/L; IQR 7.2-8.2), followed by tubercular (4.5 mmol/L; IQR 4.2-5.2) and malignant effusions (1.82 mmol/L; IQR 1.5-7.5) (p<0.01). A pleural lactate cut-off value >7 mmol/L effectively identified parapneumonic/complicated effusions. The mean P/S lactate ratio was significantly higher in tubercular effusions compared to non-tubercular effusions (5.29±2.17 vs. 3.20±2.34; p<0.01). Conclusion: Pleural fluid lactate and the P/S lactate ratio offer useful adjunctive information in the evaluation of exudative pleural effusions. While absolute lactate values assist in identifying parapneumonic effusions, the P/S ratio provides additional value in differentiating tubercular from non-tubercular causes. These parameters may improve diagnostic efficiency, particularly in resource-constrained settings.