Cytohistological correlation of pleural fluids using the international system for reporting serous fluid cytopathology: A single-center experience

Objectives: The cytologic examination of serous effusions is a widely used, inexpensive, and minimally invasive diagnostic method with high sensitivity for detecting malignancy. However, there are limited studies evaluating the validity, cytohistological correlation, and diagnostic performance of the recently proposed International System for Reporting Serous Fluid Cytopathology (TIS) in pleural effusions. This study aimed to reclassify pleural effusion cytology specimens according to the TIS system, determine the risk of malignancy (ROM) for each category, and assess the diagnostic correlation with histopathological findings. Material and Methods: A total of 167 pleural effusion cases with both cytologic and histopathologic diagnoses, evaluated between 2011 and 2023, were included. Cytologic specimens were reclassified according to TIS categories as: Non-diagnostic (ND), negative for malignancy (NFM), atypia of undetermined significance (AUS), suspicious for malignancy (SFM), and malignant (MAL). The results were compared with those obtained from our previous institutional reporting system (ND/Inadequate, NFM, SFM, MAL) and correlated with biopsy findings. Results: Based on our institutional system, the diagnostic distribution was 61.1% NFM, 20.4% MAL, 16.2% SFM, and 2.4% ND/inadequate. According to the TIS classification, 59.9% were NFM, 21.6% MAL, 11.4% SFM, 4.8% AUS, and 2.4% ND. Histopathology confirmed 50.9% benign and 49.1% MAL cases. A statistically significant correlation was observed between the previous reporting system and TIS ( P <0.001), as well as between cytological and histological results ( P <0.001). The ROM was 25% for AUS and 89.5% for SFM. Conclusion: Adequate cytologic sampling remains crucial for diagnostic accuracy in pleural effusions. The TIS provides a standardized, reproducible, and user-friendly classification scheme that facilitates communication and reduces false-positive and false-negative interpretations. Our findings support the diagnostic value of the TIS system and emphasize the importance of adopting internationally standardized reporting systems for consistent patient management.