Immunoperoxidase demonstration of carcinoembryonic antigen in pleural and peritoneal effusions.

Ninety-eight consecutive pleural or peritoneal effusions from 76 patients with various malignant diseases were studied by cytomorphologic evaluation and immunoperoxidase staining for carcinoembryonic antigen (CEA). Cells easily identified as mesothelial cells were always negative in the immunoperoxidase stain. Sixty-seven effusions were from patients with carcinoma. By cytomorphology, 32 of these 67 effusions contained malignant epithelial cells, 15 were suspect, and the remaining 20 were negative. By immunoperoxidase staining, 35 of the 67 effusions contained cells with a positive intracellular reaction for CEA; 13 of these were among the cytomorphologically suspect or negative cases. None of the 31 effusions from patients with nonepithelial malignancies showed positive immunostaining for CEA. A positive immunoperoxidase stain for CEA can thus be used to confirm the presence of malignant epithelial tumor cells in effusions when the cytomorphologic evaluation is uncertain or negative.