S159 Diagnostic Utility of Biliary Brush Cytology Compared to Fluorescence in Situ Hybridization in Assessment of Indeterminate Biliary Strictures: A Single Center Retrospective Study

Introduction: Biliary brush cytology is the current standard of care for elucidating the etiology biliary strictures; however, its diagnostic utility is limited by low sensitivity and frequently broad atypical histological interpretations, particularly in the presence of biliary stents. Fluorescence in situ hybridization (FISH) has also been identified in recent times as a valuable adjunctive tool in such clinical scenarios. This study aims to assess the diagnostic performance of FISH and compare its effectiveness with that of conventional biliary brush cytology. Methods: Charts from March 2021 to May 2024 for all bile duct brush cytology cases with follow-up surgical pathology were reviewed. Cytologic diagnoses were classified as negative, atypical, suspicious, or malignant. Correlated surgical pathology diagnoses were classified as benign or malignant and served as the confirmatory test. FISH test results were obtained for a subset of the total cytology cases. The sensitivity and specificity in identifying malignancy for cytology alone, FISH alone, and combined cytology with FISH were calculated. Results: A total of 91 cases with available biliary cytology and corresponding tissue histopathologic diagnoses were included in the analysis. Cytologic interpretations were categorized as atypical in 55 cases (60%), malignant in 27 (30%), negative in 8 (9%), and nondiagnostic in 1 (1%). Histologic evaluation confirmed malignancy in 68 cases (75%) and while 23 cases (25%) were negative for malignancy. FISH testing was performed on all 91 cases. Of these, 45 (49%) were positive, 42 (46%) were negative, and 4 (4%) were nondiagnostic. Conclusion: Consistent with prior studies, our findings demonstrate that biliary brush cytology has limited utility in confidently excluding malignancy in biliary strictures due to its low negative predictive value. Although both cytology and FISH showed excellent positive predictive values, FISH was notably more sensitive in detecting malignancy. These results contribute to the growing body of evidence that FISH can enhance diagnostic yield when coupled with cytology but highlight the need for additional diagnostic modalities, such as EUS-guided tissue acquisition, in cases with negative or indeterminate cytology.