The cytological dilemma of distinguishing malignant cells from proliferating mesothelium has generated volumes of research but no clear consensus regarding an optimal staining panel. nm23 has been frequently described in malignant cells as a metastasis suppressor, but its mechanism of action and the relationship between nm23 expression, metastases, and prognosis is controversial. This is the first study to apply nm23 immunostaining in the setting of effusion cytology. One hundred samples of effusions (56 malignant and 44 benign) were immunostained with nm23 using the biotin-avidin technique and diaminobenzidine as a chromogen. Additionally, a mucicarmine stain was performed on most samples, all of which were evaluated for nm23 expression and mucin in a blinded fashion. After all the samples were reviewed, the diagnoses were disclosed and staining patterns evaluated. From the malignant cases, 51 of 56 cases were positive for nm23 in at least 5% of malignant cells, while 27 of 44 reactive mesothelium cases were also positive in at least 5% of cells. Of the malignant cases, all non-adenocarcinomas expressed nm23. We conclude that nm23 is a highly sensitive marker for detecting malignant cells. Its relatively high rate of expression in reactive mesothelium supports previous studies that suggest nm23 is expressed in proliferating cells.

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