Introduction: Body cavity effusions are routinely used as cytologic specimens. The distinction between metastatic carcinoma, mesothelioma, and reactive mesothelial cells remains a major challenge. Immunohistochemistry (IHC) is a supplemental method that can aid in diagnosis and often involves many markers as part of an IHC panel. Several immunohistochemical markers are now widely used. This study aims to determine the optimal immunomarkers and IHC panels to differentiate reactive mesothelial cells from metastatic cancer in body cavity fluid samples. Methods: IHC was performed for claudin-4, MOC-31, Ber-Ep4, D2-40, and calretinin on sections derived from 152 cellblocks containing effusions. The samples consisted of 16 (10.53%) benign and 136 (89.47%) malignant tumors, including 87 (63.97%) lung cancers, nine (6.62%) breast cancers, 11 (8.09%) gynecologic cancers, seven (5.15%) pancreaticobiliary cancers, and 22 (16.17%) unspecified primary malignancies. Results: Claudin-4, MOC-31, Ber-EP4, D2-40, and calretinin demonstrated sensitivities of 91.18%, 91.91%, 55.88%, 90.44%, and 98.53%, respectively. The corresponding specificities were 100.00%, 100.00%, 100.00%, 93.75%, and 100.00%. The sensitivity and specificity were both 100% when claudin-4 or MOC-31 was combined with calretinin. The combination of four markers as an IHC panel (claudin-4, MOC-31, calretinin, and D2-40) had a sensitivity of 97.79% and a specificity of 100.00%. Conclusion: Claudin-4 and MOC-31 both demonstrated significant diagnostic value in distinguishing metastatic epithelial carcinoma from reactive mesothelium. The sensitivity, specificity, and accuracy of these two markers, one of which is an epithelial marker and one of which is a mesothelial marker, reached 100%. Therefore, a combination of these two markers may be appropriate.

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