The Role of Immunohistochemistry in Reducing the "Atypia of Undetermined Significance (AUS)" Category in Serous Fluid Cytology.

Background and Aims: The International Serous Fluid Cytopathology Reporting System aimed to establish standardized protocols to ensure consistency in the reporting of serous fluid cytological specimens. In the search for higher diagnostic accuracy and a reduction in indeterminate categories, such as atypia of undetermined significance (AUS), ancillary tests like immunohistochemical (IHC) staining panels were performed. In our study, we aimed to evaluate whether the category of cases diagnosed as AUS by initial examination would change at the end of IHC studies. Materials and Methods: In total, 375 serous fluid cytology samples were examined in our laboratory for 10 months. Of these, 37 cases that were initially diagnosed as AUS were included in the study. A control group, comprising 20 cases initially diagnosed as negative for malignancy (NFM) was used. For the IHC study, sections from cell blocks were used for each group Then, the slides were stained with Ep-CAM/epithelial specific antigen (MOC31), Hector Battifora mesothelial-1 (HBME-1), and cluster of differentiation 68 (CD68). Results: Following the IHC study involving MOC31, HBME-1, and CD68, a significant reclassification was observed in cases initially diagnosed as AUS. Specifically, in 86.1% of these cases, a definitive categorization into either NFM or malignant was achieved. Statistical analysis revealed a significant difference between the two groups in terms of achieving a definitive category after the IHC study (P < 0.05). Conclusion: Our study emphasizes the critical importance of enhancing the initial IHC panel, initially composed of epithelial and mesothelial markers, with CD68. This strategic addition contributed significantly to the reduction of cases categorized as AUS. [ABSTRACT FROM AUTHOR]