The Role of Pleural Fluid C-Reactive Protein in the Diagnosis of Exudative Pleural Effusions

Background and objective Pleural effusion develops when there is disequilibrium between pleural fluid formation and absorption. Light's criteria are currently used to differentiate transudative from exudative effusion. If the pleural effusion is exudative, it requires extensive diagnostic workup to identify the local cause of the effusion. Pleural fluid cell count and differentials, glucose level, adenosine deaminase (ADA), fluid GeneXpert for Mycobacterium tuberculosis (MTb), fluid culture, and cytology are currently used for further evaluation of exudative pleural effusions. However, the sensitivity and specificity of the above tests are not dependable. The pleural fluid C-reactive protein (CRP) is likely to reflect serum CRP levels because the CRP in the pleural fluid may be caused by increased diffusion from the blood due to inflamed capillary leakage. In this study, we aimed to examine the role of pleural fluid CRP levels in the differential diagnosis of exudative effusion. Materials and methods Based on Light's criteria, this study included 100 patients with exudative pleural effusion. Serum CRP and pleural fluid CRP were assessed with the CRP-Turbilatex-quantitative turbidometric immunoassay method based on the principle of an agglutination reaction. Receiver operating characteristic (ROC) curves were generated by plotting sensitivity against 1-specificity, and the area under the curve (AUC) with a 95% confidence interval (CI) was calculated. After data collection, statistical analysis was performed using SPSS Statistics v28.0 (IBM, Armonk, NY). Results Our study showed a significant difference in pleural fluid CRP levels (p<0.001). Pleural fluid CRP was significantly higher in the empyema and parapneumonic groups compared to tuberculous and malignant effusions. The optimal cut-off value of CRP ≥47.4 mg/dl yielded 87.5% sensitivity and 92.5% specificity in differentiating parapneumonic effusion from tuberculous effusion. Pleural fluid CRP proved to be an excellent marker for distinguishing parapneumonic effusion from malignancy (cut-off value ≥49.2 mg/dl, 75% sensitivity, and 85.7% specificity) and parapneumonic plus empyema from tuberculous effusion plus malignant effusion (cut-off value ≥47.4 mg/dl, 84.6% sensitivity, and 90.8% specificity). Conclusion Pleural fluid CRP levels can be used as an additional tool in the differential diagnosis of exudative effusion. It significantly differentiates parapneumonic effusion and empyema from tuberculous and malignant effusions.