Use of the monoclonal antibody MOC-31 as an immunomarker for detecting metastatic adenocarcinoma in effusion cytology

BACKGROUND: MOC-31 is an established immunologic marker with which to detect adenocarcinomas. The objective of the current study was to evaluate the use of MOC-31 in the diagnosis of metastatic adenocarcinoma in effusion specimens. METHODS: The authors evaluated cytologic specimens of effusions/washings in which MOC-31 immunostaining was performed on unstained cell block sections or Papanicolaou-stained cytospin preparations. Membranous staining with or without cytoplasmic staining was considered to be positive. The immunostaining results were correlated with the cytologic diagnoses and clinical follow-up data. RESULTS: A total of 215 effusions and washings were identified (cell blocks in 162 cases, cytospin preparations in 53 cases, and both in 2 cases in which MOC-31 immunostaining was performed). A total of 94 (44%) of the 215 cases were found to be positive for malignancy, including 87 metastatic adenocarcinomas. Specimens were positive for MOC-31 in 76 (87%; 55 cell blocks and 21 cytospin preparations) of 87 cases of metastatic adenocarcinoma. Eleven cases of metastatic adenocarcinoma were found to be negative for MOC-31 (4 cases from lung tumors, 2 from stomach tumors, 2 from colon tumors, 2 from breast tumors, and 1 from a renal tumor). Minimal and/or focal cytoplasmic staining for MOC-31 was noted in 13% of cases of reactive mesothelial cells/mesothelioma. The sensitivity of MOC-31 for metastatic adenocarcinoma was 89%, the specificity was 100%, the negative predictive value was 92%, and the positive predictive value was 100%. CONCLUSIONS: MOC-31 alone was found to be highly sensitive for distinguishing reactive mesothelial cells/mesothelioma from metastatic adenocarcinoma in effusion specimens. Interpreting membranous MOC-31 staining as positive can help prevent confusion between reactive mesothelial cells/mesothelioma and metastatic adenocarcinoma.