MOC‐31 aids in the differentiation between adenocarcinoma and reactive mesothelial cells

BACKGROUND Evaluation of effusion specimens for the presence of adenocarcinoma often is complicated by the presence of reactive mesothelial cells that can mimic adenocarcinoma. Ancillary studies, in particular immunohistochemistry, can be helpful in making this distinction. MOC‐31 is an antibody that recently was reported to be useful in distinguishing adenocarcinoma from mesothelioma in tissue specimens. In this study we examined the utility of this antibody in pleural effusions. METHODS Eighty‐nine archival, formalin fixed, paraffin embedded cell blocks representing 59 adenocarcinomas, 12 other neoplasms (including 6 mesotheliomas), and 18 reactive effusions were retrieved. After protease digestion, recut slides were immunostained with the MOC‐31 antibody utilizing a modified avidin‐biotin complex technique. Only membrane‐based reactivity was considered as positive. RESULTS In two adenocarcinomas there was insufficient material remaining in the cell block. Among the 57 remaining cases, reactivity was observed in 54 cases. Reactivity also was observed in one of six mesotheliomas and one small cell carcinoma. The remaining cases, including all 18 reactive effusions, were nonreactive. In distinguishing adenocarcinoma from reactive mesothelial cells, the presence of MOC‐31 reactivity was found to be 95% sensitive and 100% specific with a positive predictive value of 100% and a negative predictive value of 95%. CONCLUSIONS MOC‐31 is useful in differentiating between adenocarcinoma and reactive mesothelial cells in pleural effusion specimens. Cancer (Cancer Cytopathol) 1999;87:390–4. © 1999 American Cancer Society. The antibody MOC‐31 is helpful in distinguishing metastatic adenocarcinoma from reactive mesothelial cells in pleural effusions.