Real-word Impact of the 2023 Bethesda System on Thyroid FNA CategoryDistribution: A Multi-phase Longitudinal Study From Algeria

BACKGROUND: The third edition of The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC, 2023) introduced refinements in the criteria for indeterminate thyroid lesions. Observed temporal shifts in cytologic categories may reflect both classification changes and evolving technical and interpretive practices. This study evaluated the relative contributions of these factors in a real-world hospital setting. METHODS: We conducted a retrospective study of 731 ultrasound-guided thyroid fine-needle aspiration (FNA) specimens at a public referral center in western Algeria across four periods: 2017, 2021, 2024, and 2025. Cases from 2017 and 2021 were reported using TBSRTC 2017, while 2024-2025 cases used TBSRTC 2023. Category distributions were compared using chi-square and two-proportion z-tests (p < 0.05). RESULTS: Overall category distribution differed significantly over time (χ² = 61.0, p < 0.0001). Non-diagnostic (Category I) rates declined from 20.5% in 2017 to 7.8% in 2021 (p < 0.001), reflecting technical improvements, and remained stable thereafter (11.7-12.3%). AUS/FLUS (Category III) rates were stable (4.9-7.1%; p > 0.3). Category IV (FN/SFN) transiently increased in 2021 (20.4%) before stabilizing post-2023 implementation (3.1-5.1%; p < 0.0001 for 2021 vs 2024). Categories V-VI remained consistently low (≤4.9% and ≤0.4%, respectively) without evidence of systematic under-calling. CONCLUSION: Temporal variations in thyroid cytology reflected the combined effects of technical maturation, institutional adaptation, and classification updates rather than TBSRTC 2023 alone. Multi-phase longitudinal analyses are critical to separate these influences when evaluating revised reporting systems, particularly in resource-limited settings.