The distinction of adenocarcinoma from malignant mesothelioma in cell blocks of effusions: the role of routine mucin histochemistry and immunohistochemical assessment of carcinoembryonic antigen, keratin proteins, epithelial membrane antigen, and milk fat globule-derived antigen.

The immunohistochemical profile (i.e., carcinoembryonic antigen, keratin proteins, epithelial membrane antigen, human milk fat globule-derived antigen, and mucin) of paraffin-embedded cell blocks of 20 malignant effusions from patients with malignant mesothelioma was compared with that of 39 malignant effusions from patients with metastatic adenocarcinoma to determine whether these markers distinguished between these tumor types. Twenty-three adenocarcinomas (59 per cent) stained for mucin. Immunoreactivity for carcinoembryonic antigen (CEA) was observed in 28 adenocarcinomas (72 per cent). All were immunoreactive for keratin proteins, and 29 adenocarcinomas (74 per cent), including seven that were mucin and CEA negative and exhibited a "peripheral predominant" staining pattern for keratin proteins. By contrast, none of the mesotheliomas stained for mucin or for CEA, and, although all were immunoreactive for keratin proteins, none demonstrated a peripheral predominant pattern of staining. Epithelial membrane antigen and milk fat globule-derived antigen were identified in the majority of both mesotheliomas and adenocarcinomas. Neither staining intensity nor pattern of reactivity of these markers clearly distinguished the tumors. This study of cell blocks of serous effusions suggests that staining for mucin, immunoreactivity for carcinoembryonic antigen, and a peripheral predominant pattern of reactivity for keratin proteins represent highly characteristic markers of adenocarcinomas, which identify the majority of these tumors (38 of 39) and allow their distinction from malignant mesotheliomas.