Immunocytochemical studies of epithelial markers in pleural and peritoneal effusions as an adjunct to cytologic diagnosis

Monoclonal antibodies were used in an indirect immunoperoxidase assay to investigate 83 smears of pleural or peritoneal effusions for expression of the epithelial markers MAM-6, MAM-3, and carcinoembryonic antigen (CEA). All but one smears containing tumor cells according to the evaluation of H&E preparations were positive for one, two or all three markers, the exception being a malignant melanoma. Three of 5 cases, suspicious by routine cytology, exhibited marker expression in a different number of cells and thus confirmed the cytological diagnosis. Five of 63 cytologically negative smears exhibited single cells or small cell clusters positive for up to three markers. Four of these patients were found to suffer from metastatic cancer, as established by clinical follow-up and subsequent biopsy or autopsy, respectively. Felty's syndrome and concomitant serositis were diagnosed and confirmed by autopsy in one of the MAM-6 positive cases with negative cytology. The comparatively strong MAM-6 expression in some mesothelial cells of this patient might have been induced by abnormal stimulation due to the rheumatic disease. The results of this study encourage wider use of immunocytochemistry as an adjunct to cytological diagnosis in effusions.