A single tertiary institution review of the international system for serous fluid cytopathology and the impact of cell block and ancillary studies on its performance

We sought to assess the utility of the International System for Serous Fluid Cytopathology (TIS) in the context of our department’s routine practice. We examined 1028 archived effusion cytology (pleural, peritoneal, and pericardial) cases from 2018 to 2019, and re-classified them along the international system into the following diagnostic categories: nondiagnostic (ND), negative for malignancy (NFM), atypia cells of undetermined significance (AUS), suspicious for malignancy (SFM), and malignant (MAL). The full distribution of the cases examined was as follows: ND 2.0%; NFM 66.1%; AUS 6.0%; SFM 4.7%; MAL 21.2%. Overall risk of malignancy for each category was calculated as: ND 30.0%; NFM 18.0%; AUS 61.9%; SFM 100%; MAL 94.4%. The overall performance attributes of TIS were as follows: sensitivity 57.1%; specificity 98.3%; positive predictive value 94.4%; negative predictive value 82.0%; diagnostic accuracy 84.5%. The new classification was simple and intuitive to use and our results appear to fall within the expected ranges of the new guidelines, with risk of malignancy and accuracy comparable to similar studies. The availability of a cell block allowed for refinement of the diagnosis in a majority of cases with equivocal cytology, though this was dependent on the cell yield. •We have undertaken one of the largest scale assessments of the International System for Serous Fluid Cytopathology (TIS) with over 1000 archival cases reviewed and re-categorized.•Being a relatively new system, we believe that our study assists in the validation of the TIS, as well as contributing to a still-developing consensus on parameters such as expected rates and risk of malignancy for each diagnostic category.•We are one of the first groups to compare our archival cell block results with the TIS, and demonstrate their predictive value, particularly in the more equivocal atypical and suspicious categories.