Pleural fluid biomarkers for the diagnosis and management of malignant pleural effusions: a clinical review

Initial pleural fluid analysis is a fundamental step in the evaluation of suspected malignant pleural effusion (MPE). Most MPEs present as exudates, often hemorrhagic, with mononuclear cell predominance. Basic biochemical parameters-glucose, total protein, LDH, ADA, and pH-help distinguish MPE from other causes and offer prognostic information. Low glucose and pH, and elevated LDH, are associated with higher tumor burden and poorer outcomes. Flow cytometry can detect high-fluorescence cells suggestive of malignancy, while additional markers like CRP, cholesterol, amylase, and lipids provide complementary diagnostic value, especially when interpreted alongside tumor markers (TMs). Among TMs in pleural fluid, CEA is the most validated and widely used, showing high specificity for MPE. Others-such as CA 15.3, CYFRA 21-1, CA 125, CA 19.9, NSE, and HE4-offer variable sensitivity and specificity depending on tumor type and clinical context. False positives can occur in benign or inflammatory conditions, emphasizing the need for cautious interpretation. Other cancer biomarkers in pleural fluid-such as VEGF, Apolipoprotein E, calprotectin, endostatin, and homocysteine-may enhance diagnostic and prognostic capabilities. VEGF and endostatin reflect tumor angiogenesis and may also serve as therapeutic targets, while homocysteine shows promise in detecting MPEs not identified by conventional TMs. Multimarker strategies significantly improve diagnostic accuracy. Combinations of two pleural fluid TMs, such as CEA with CA 15.3, or diagnostic models like the MPER score (CEA + homocysteine), have shown excellent performance. Panels with three or more markers, including inflammatory or metabolic biomarkers (ADA, CRP, and %polymorphonuclear leukocytes) further enhance sensitivity and specificity. Molecular analysis in pleural fluid has emerged as a promising approach for the diagnosis of MPE, enabling the detection of mRNA, DNA methylation patterns, lncRNAs, miRNAs, or circulating tumor DNA. Although these biomarkers have demonstrated good diagnostic accuracy, they are not yet implemented in routine clinical practice, and most studies have primarily focused on MPE due to lung cancer. In malignant pleural mesothelioma, where cytology has limited sensitivity, the most extensively investigated markers in pleural fluid are mesothelin and fibulin-3. Among conventional tumor markers, the pleural fluid CYFRA 21-1/CEA ratio has shown high diagnostic accuracy, further enhanced when combined with mesothelin. Pleural fluid fibulin-3 has also been identified as an independent prognostic factor for survival.