Tingting Liu ^1,2 Lin Deng ^2* Hongming Lin ^2* Ruohan Su ^2* Zhiqing Lin ^2* Hu Zhao ^2* Sheng Huang ^2*

• ¹ The 900th Hospital of the People’s Liberation Army Joint Service Support Force, Fuzhou, China
• ² Department of General Surgery, 900 Hospital of the Joint Logistics Team of the Chinese PLA, Fuzhou, Fujian Province, China

Objective: To explore the diagnostic value of fine-needle aspiration cytology and fine-needle aspirate eluent thyroglobulin (FNA-Tg) test in the diagnosis of lateral neck lymph node metastasis (LLNM) in papillary thyroid carcinoma (PTC). Methods: Prospectively collect data of 215 PTC patients who underwent lateral neck lymph node dissection at the 900th Hospital from October 2022 to June 2024 were included, with 249 suspicious lymph nodes. To compare the diagnostic value of FNA-Tg and FNAC in overall and different lymph node sizes, and determine the optimal threshold, explore reliable diagnostic criteria. Results: Overall analysis showed that FNA-Tg had a higher diagnostic efficacy for LLNM in PTC patients compared to FNAC (AUC=0.950 vs 0.766), with the optimal diagnostic threshold of 16.45 μg/L for the primary LLNM group and 0.15 μg/L for the recurrent LLNM group. Subgroup analysis with paired-2 test showed that there was a statistically significant difference in the diagnostic value of FNA-Tg and FNAC for LLNM in PTC patients only when the lymph node short diameter was ≤0.8cm (P<0.001), while there was no statistical difference when >0.8cm (P=0.064). Further exploration of the diagnostic criteria for FNA-Tg, combined with the optimal threshold value of FNA-Tg and the FNA-Tg/sTg ratio, yielded the best diagnostic criteria ③(AUC=0.942). Conclusion: FNA-Tg has a higher diagnostic value for PTC LLNM than FNAC, with optimal thresholds of 16.45 μg/L (primary LLNM group) and 0.15 μg/L (recurrent LLNM group), especially when the suspicious lymph node short diameter is ≤0.8cm, FNA-Tg may be considered as the preferred option.