Comparison of Serum and Pleural Fluid Neutrophil-to-Lymphocyte Ratios in Tuberculous, Parapneumonic, and Lung Cancer–Associated Exudative Pleural Effusions

Abstract

Neutrophil-to-lymphocyte ratio (NLR) is an inflammatory biomarker, but evidence comparing serum and pleural fluid NLR in pleural effusion is limited. This study compared NLR in 210 patients with exudative pleural effusion (70 tuberculous, 70 parapneumonic, and 70 lung cancer–associated pleural effusions) treated between January and December 2025. Serum NLR was calculated from blood neutrophil and lymphocyte counts and pleural fluid NLR from the polymorphonuclear-to-mononuclear cell ratio. Comparisons, regression, and receiver operating characteristic (ROC) analyses were performed. Serum NLR exceeded pleural fluid NLR (7.40 vs. 0.67, p<0.001). Serum NLR did not differ among etiologies (all p>0.05), but pleural fluid NLR was highest in parapneumonic pleural effusion, intermediate in lung cancer–associated effusion, and lowest in tuberculous effusion (all p<0.001). Serum and pleural fluid NLR were uncorrelated. Parapneumonic pleural effusion independently predicted higher pleural fluid NLR (β=14.93, p=0.001). Pleural fluid NLR showed performance for tuberculous pleural effusion (AUC=0.947; cut-off ≤0.43) and parapneumonic effusion (AUC=0.991; cut-off ≥1.04); serum NLR showed poor discrimination. Pleural fluid NLR outperformed serum NLR in differentiating exudative pleural effusion etiologies, particularly tuberculous versus parapneumonic effusions.