Laboratory Discrimination Between Neutrophilic Malignant and Parapneumonic Pleural Effusions

Background: Malignant pleural effusion (MPE) occasionally demonstrates neutrophilic predominance, commonly found in parapneumonic pleural effusion (PPE). In comparison with lymphocytic MPE, neutrophilic MPE may have different characteristics associated with a more intense inflammatory response and poor prognosis. These characteristics of neutrophilic MPE may lead to inappropriate management and delayed diagnosis. Moreover, the limited diagnostic yield of microbiologic and cytologic tests makes early differential diagnosis between neutrophilic MPE and PPE more challenging. This study investigated objective laboratory findings to help distinguish neutrophilic MPE from PPE. Materials and Methods: A retrospective study was conducted on patients with neutrophilic MPE and PPE. Routine blood and pleural fluid data of the 2 groups were compared, and the diagnostic performances of predictors for neutrophilic MPE were assessed using receiver-operating characteristic curves. Results: Forty-one and 140 patients with neutrophilic MPE and PPE, respectively, were included. In final analysis, serum C-reactive protein, pleural fluid neutrophil-to-lymphocyte ratio, and pleural fluid carcinoembryonic antigen were significantly different between the 2 groups. With cut-off values of C-reactive protein <6.0 mg/dL, neutrophil-to-lymphocyte ratio <3.0 and carcinoembryonic antigen >8.0 ng/mL, the presence of any 2 or more parameters provided an area under the curve of 0.928 (95% CI, 0.851-0.999), yielding a sensitivity of 88%, specificity of 98%, positive predictive value of 92% and negative predictive value of 96% for identifying MPE. Conclusions: MPE should be considered even in patients with neutrophilic exudative effusion, especially if at least 1 predictor for neutrophilic MPE is present. Our results may help guide differentiation of neutrophilic MPE from PPE.