Lung adenocarcinoma (LADC) is the most common occult primary in patients presenting with a malignant pleural effusion. Distinguishing metastatic LADC from reactive mesothelial cells (RMC) and malignant mesothelioma (MM) based on morphology alone has been a persistent diagnostic challenge in cytopathology. Claudin-4, a major functional constituent of tight junctions, has been shown to help distinguish LADC from RMC and MM in surgical specimens. Our goal was to further validate and assess the utility of Claudin-4 in comparison to BerEP4 and B72.3 in malignant effusions with a focus on metastatic LADC.We evaluated 58 pleural effusions (40 LADC, 10 RCM, and 8 MM). Immunohistochemistry was performed using Claudin-4, Ber-EP4, and B72.3 on cell blocks. Staining patterns, quantity of tumor cells, and intensity of staining (weak, moderate, or strong) were assessed.All cases of LADC were positive for Claudin-4 with an overall sensitivity of 100% (40 of 40) and specificity of 100% (18 of 18). In addition, Claudin-4 showed the highest quantity and quality of staining with 3+ staining intensity in 73% (29 of 40) of cases, compared with 35% (14 of 40) of cases using BerEP4 and 52% (21 of 40) of cases using B72.3. The sensitivity and specificity for BerEP4 were 90% and 78%, respectively. The sensitivity and specificity for B72.3 were 87.5% and 100%, respectively.In this study, Claudin-4 performed superiorly compared to BerEP4 and B72.3 in distinguishing lung adenocarcinoma from RMC or MM in pleural effusions. Our results show Claudin-4 is a useful marker for distinguishing RMC and MM from lung adenocarcinoma, with high sensitivity (100%) and specificity (100%), compatible with studies shown in the literature.