Supplementary Material for: Evaluating the Impact of Sample Volume on Cytological Diagnosis of Pleural Effusion: A Single-Institution Study Using the International System for Reporting Serous Fluid Cytopathology

Introduction: Malignant pleural effusion is a frequent manifestation in cancer patients, with effusion cytology playing a vital role in diagnosis and subtyping. Present study evaluated the effect of sample volume on malignancy detection and estimated the risk of malignancy (ROM) by using the International System for Reporting Serous Fluid Cytopathology (TIS).
Methods: Pleural effusions submitted from May 2021 to December 2022 were reclassified using the International System for Reporting Serous Fluid Cytopathology (TIS) into five categories: non-diagnostic (ND), negative for malignancy (NFM), atypia of uncertain significance (AUS), suspicious for malignancy (SFM), and malignant (MAL). Risk of malignancy (ROM) and performance metrics were calculated based on follow-up histology and/or repeat cytology, ancillary tests and clinico-radiology. Volume data from 493 samples were grouped into six bins (0–10 mL to >300 mL), and malignancy fractions were analysed. Generalized Estimating Equations (GEE) logistic regression assessed the impact of volume, sex, and age on diagnostic outcomes. Analysis was performed in R.
Results: Of 1265 samples from 1107 patients, 875 (69.2%) had follow-up data. ROM estimates were: ND 23.6%, NFM 11.6%, AUS 57.1%, SFM 100%, MAL 97.5%. MAL samples had significantly higher median volume than NFM (100 vs 35 mL; p<0.000). False negatives had lower volumes than true positives (50 vs 80 mL; p=0.027). Malignancy detection was lowest in samples <10 mL (7.4%) and highest in >300 mL (40.4%). Volumes <25 mL were significantly associated with reduced odds of malignancy detection (p<0.05).
Conclusion: Sample volumes <25 mL are linked to lower malignancy detection, underscoring the importance of adequate volume and supporting TIS implementation in routine cytology.