Diagnostic utility of immunocytochemistry by using liquid-based cytology (LBC) slides.

Introduction: Cytological specimens, such as fine needle aspirations (FNAs) and exfoliative cytology samples, are minimally invasive options for diagnostic purposes. Liquid-based cytology (LBC), originally designed for cervical cytology, has gained prominence as an alternative technique for non-gynecological cytology. Immunocytochemistry (ICC) is an ancillary method used when diagnosis becomes challenging due to morphological overlap or the need for cellular origin clarification. This study aims to assess the diagnostic utility of ICC when applied to LBC slides and evaluate its effectiveness in relation to the waiting (lag) time of residual material.

Materials and methods: A total of 74 cases in which ICC was applied to LBC slides were studied over one year in a reference pathology laboratory (Prof. Dr. Cemil Tascioglu, Pathology Laboratory, City Hospital, Istanbul, Turkey). Cases in which immunohistochemistry (IHC) was performed on formalin-fixed paraffin-embedded cell blocks were excluded. The SurePath PAP method was used for the main LBC cytology slides. For the ICC study, 1-4 PAP-stained LBC slides were obtained from each case's residual material and stained with a primary antibody.

Results: The positive immunostaining was obtained in 81% of cases. The samples were categorized into groups based on the waiting time of residual LBC material for ICC analysis: 1-5 days, 6-10 days, 11-20 days, and 21-38 days. Comparative analysis revealed a decline in ICC efficacy as the waiting (lag) time increased. Additionally, a statistically significant difference was observed between the 11-20 days and 21-38 days groups (P < 0.05). An analysis of 142 LBC slides stained by ICC revealed that nuclear markers exhibited higher positivity compared to non-nuclear markers, although no significant difference was detected between the two groups.

Conclusions: High positivity rates can be obtained in ICC studies performed on additional slides obtained from residual LBC material within the first 20 days. ICC applied to LBC slides is an important and useful alternative for diagnostic and prognostic markers in cases without a cell block or with a cell block without sufficient number of cells.