Diagnostic Value of Claudin-4 and EZH2 Immunohistochemistry in Effusion Cytology

BACKGROUND: Metastatic adenocarcinoma (MAC) accounts for most cases of malignant effusions. Sometimes, it can be difficult to distinguish MAC from reactive mesothelial cells (RMC) in cytologic specimens. Our aim was to assess the diagnostic performance of a novel immunohistochemical panel composed of claudin-4 and EZH2 in differentiating MAC from RMC in effusion cytology. METHODS: A total of 80 cases of serous effusions (48 MAC and 32 RMC) were included. Immunohistochemistry using claudin-4 and EZH2 was performed on cell block sections of these cases. Assessment of staining patterns, intensity and percentage of target cells stained was done. RESULTS: Claudin-4 showed membranous staining in 46/48 of MAC and 1/32 of RMC. High EZH2 (≥ 50% of target cells) was detected in 42/48 MAC and 2/32 RMC. For the discrimination between MAC and RMC, claudin-4 exhibited 95.8% sensitivity and 96.9% specificity, high-EZH2 exhibited 87.5% sensitivity and 93.8% specificity, while the combination of both claudin-4 and high EZH2 showed 100% sensitivity and 90.6% specificity. CONCLUSION: Claudin-4 shows high sensitivity and specificity in differentiation between MAC and RMC in effusion cytology, and might be useful as a solitary marker for MAC. Adding EZH2 to claudin-4 increases the sensitivity to 100%. However, the interpretation of EZH2 results can be challenging due to its focal expression in RMC and inflammatory cells.