Purpose: The purpose was to compare the frequency of metastatic and nonmetastatic lymph nodes diagnosed by fine needle aspiration biopsy cytology (FNAC) and thyroglobulin concentration from fine needle aspiration biopsy washout fluid (FNAB-Tg) in an indeterminate range (0.2-100 ng/mL), and to evaluate the most appropriate threshold value of FNAB-Tg in an indeterminate range. Materials and Methods: We performed ultrasound-guided FNAB and FNAB-Tg in suspicious metastatic cervical lymph nodes of papillary thyroid carcinoma and performed surgery. Ninety-five lymph nodes with indeterminate values of FNAB-Tg ranging from 0.2-100 ng/mL in ninety-two patients were included in this study. The diagnostic performances in multiple Tg levels (0.7, 1.0, 5.0, 10.0, 20.0, 50.0) were evaluated to compare with FNAB cytology using sensitivity, specificity, and accuracy with area under the curve (AUC) analysis. Results: Forty-two were metastatic lymph nodes and fifty three were nonmetastatic lymph nodes. FNAB-Tg ranged from 0.22 to 90.9 ng/mL in metastatic lymph nodes (mean; 34.3±33.3 ng/mL) and 0.20 to 56.7 ng/mL in nonmetastatic lymph nodes (mean; 4.9±11.1 ng/mL) (p<0.001). The most excellent diagnostic performance was displayed in 5 ng/mL of FNAB-Tg with AUC of 0.76, sensitivity, specificity, accuracy, 69.0, 83.0, and 76.8, respectively. However, there was no significant difference from 10 ng/mL FNAB. Conclusion: We ascertained that 5 ng/mL yielded the most excellent diagnostic performance among FNAB-Tg levels in the present setting with a large series with the indeterminate range (0.2-100 ng/mL) of FNAB-Tg values. These results need additional confirmation under different laboratory conditions.
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