Objective: The differential diagnosis of malignant pleural mesotheliomas and lung adenocarcinomas is problematic for pathologists especially in small biopsy materials. In this study, we aimed to investigate whether the immunohistochemically determined Claudin 3 and Claudin 4 immunoreactivities are useful in the differential diagnosis between malignant pleural mesotheliomas and lung adenocarcinomas.

Material and Method: Using the epitope-specific rabbit antibody Claudin 3 and Claudin 4, 32 cases of malignant pleural mesothelioma (25 epithelioid, 7 biphasic) and 14 cases of lung adenocarcinoma were studied immunohistochemically

Results: None of the cases of malignant pleural mesotheliomas expressed Claudin 4, whereas all lung carcinomas were immunopositive. Claudin 3 expression was immunonegative in 29 (90.6%) cases and immunopositive in 3 (9.3%) cases of malignant pleural mesothelioma. With Claudin 3, 11/14 (78.9%) lung adenocarcinomas and 3/32 (9.4%) malignant pleural mesotheliomas were immunopositive (p=0.000).

Conclusion: In conclusion, Claudin 4 antibody was found to be a highly sensitive and specific marker for distinguishing between malignant pleural mesothelioma and lung adenocarcinoma.