Epithelial Membrane Antigen. Its use in the cytodiagnosis of malignancy in serous effusions

The cytological distinction between reactive mesothelial and malignant cells frequently causes problems for the diagnostic cytologist. In order to determine whether an immunocytochemical method might help resolve these difficult cases, we have stained smears from 309 serous effusions from 246 patients for the Epithelial Membrane Antigen (EMA). The EMA staining was classified as strong, weak or negative. Carcinoma cells (as diagnosed by conventional cytology) stained strongly for EMA in 63 of the 116 positive smears (54%). Five out of 15 (33%) of cytologically suspicious smears from patients with known carcinomas gave a strong EMA stain. Of particular interest were three effusions in which malignant cells were not identified in conventionally stained smears and in which a small number of EMA positive cells were identified. The EMA positive cells were subsequently restained by the Papanicolaou method and identified as malignant on retrospective morphological examination.