Diagnostic Utility of The International System for Reporting Serous Fluid Cytopathology

Serous effusions are common specimens in cytopathology, yet their reporting has historically lacked uniformity. The International System for Reporting Serous Fluid Cytopathology (TIS) provides a standardized five-tiered classification system to improve diagnostic clarity and clinical communication. This study was conducted to assess the diagnostic utility and applicability of TIS by determining the risk of malignancy (ROM) and diagnostic accuracy metrics for each category.

A retrospective analysis was conducted on all serous effusions (pleural, pericardial, ascitic fluids, and peritoneal washes) reported from January 2021 to December 2023. Cases were re-categorized according to the TIS framework. The ROM for each category was calculated with diagnostic accuracy metrics, including sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV).

A total of 1,447 cases were included in the analysis. Distribution across TIS categories was as follows: nondiagnostic (ND) (8 cases; 0.6%), negative for malignancy (NFM) (1,170 cases; 80.9%), atypia of undetermined significance (AUS) (40 cases; 2.8%), suspicious for malignancy (SFM) (35 cases; 2.4%), and malignant (MAL) (194 cases; 13.4%). The calculated ROM for each category was 62.5% for ND, 28.8% for NFM, 66.7% for AUS, 93.8% for SFM, and 98.7% for MAL. The sensitivity, specificity, PPV, and NPV are 33.8%, 99.5%, 97.9%, and 69.7%, when AUS and NFM are considered negative. Sensitivity, specificity, PPV, and NPV are 38.1%, 98.1%, 92.9%, and 70.8%, when AUS, SFM, and MAL cases are considered positive.

The implementation of TIS provides a standardized framework for reporting serous effusions with definite categorization. The ROM for each category helps in risk stratification of cases reported under TIS. The system demonstrates high specificity and PPV, making it a robust tool for the detection of malignancy.