The international system for reporting serous fluid cytopathology: An institutional experience on its implication and assessment of risk of malignancy in effusion cytology

Background: The "international system for reporting serous fluid cytopathology"(TIS) consists of five diagnostic categories: nondiagnostic (ND), negative for malignancy (NFM), atypia of undetermined significance (AUS), suspicious for malignancy (SFM), and malignant (MAL). The study was conducted to reclassify effusion cytology samples according to the newly proposed TIS to calculate the risk of malignancy (ROM) for each category and to conduct performance analysis. Materials and Methods: The study was a retrospective observational study conducted at a tertiary care institution in North India. Clinical data of the cases from June 2013 to July 2021 were retrieved and the cases were reviewed by two cytopathologists. All cases were reclassified according to the proposed TIS system into five categories. Results: A total of 2318 patients were included in the study over a time span of 8 years, and 1614 (69.6%) cases of pleural effusion cytology, 612 (26.4%) cases of peritoneal effusion cytology, and 92 (3.9%) cases of pericardial effusion cytology were included. All effusion cytology smears were recategorized as per TIS guidelines into ND, NFM, AUS, SFM, and MAL with 4 (0.17%), 1756 (75.75%), 12 (0.51%), 57 (2.46%), and 489 (21.11%) cases, respectively, and ROM was 25%, 17.9%, 66.7%, 75.4%, and 96.5%, respectively, for the categories. Conclusion: Besides being a simple, easy, and user-friendly system, TIS has the benefit of risk stratification and ROM for each category. The TIS system proposed a tiered scheme, which places the effusion cytology into well-defined categories, and therefore has lesser chances of false-positive and false-negative cases.