The cytologic diagnosis of malignant cells in serous effusions can be difficult. A wide variety of immunostains and other diagnostic techniques have been studied but without widespread acceptance of one staining panel or technique. Expression of the tyrosine kinase c-Met has been associated with several malignancies but not with benign mesothelium. We investigated the diagnostic value of the c-Met immunostain in serous effusions. Cell block material from 76 cases of unequivocally benign or malignant effusions were studied. Cases were stained with c-Met using the avidin-biotin complex method following antigen retrieval. The presence of strong granular cytoplasmic staining that was distinct from background staining was considered positive. Positive cells were identified in 38 of 42 (90%) malignant cases and in 18 of 34 (53%) benign cases. Typically, benign cases contained only individual positive cells, but positive cell clusters were also identified. The c-Met immunostain lacks sufficient specificity to be clinically useful in this cytologic setting. The expression of c-Met in benign mesothelium may reflect mesothelial proliferation.

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