CONTEXT There are still some controversies regarding the cutoff value and the influential factors of thyroglobulin (Tg) concentration in washout fluid from fine-needle aspiration (FNA) biopsy (FNA-Tg) on cervical lymph nodes (LNs) in patients with papillary thyroid cancer (PTC).

OBJECTIVE Our aims were to validate the cutoff value of FNA-Tg in diagnosing malignant LNs on a large scale and to investigate the influential factors that could result in the discrepancy between the final diagnosis and FNA-Tg.

DESIGN, SETTING, AND PARTICIPANTS We conducted a retrospective cohort study based on hospital records with 528 cases of FNA-Tg measurement from 419 PTC patients.

MAIN OUTCOME MEASURE The cutoff value of FNA-Tg was obtained from receiver operating characteristic analysis with final diagnosis. Binary logistic regression analysis was performed to investigate the influential factors.

RESULTS In the final diagnosis, 190 LNs were malignant, and 338 LNs were benign. The median FNA-Tg was 521.2 (3676.8) ng/mL in malignant LNs, and 0.1 (0.2) ng/mL in benign LNs. The optimal cutoff value of FNA-Tg in distinguishing malignant LNs from benign LNs was 1.0 ng/mL (sensitivity, 93.2%; specificity, 95.9%) in all cases. Combining FNA-Tg and FNA cytology showed superior diagnostic power (sensitivity, 98.4%; specificity, 94.4%) when compared with diagnostic strategy using either FNA cytology or FNA-Tg alone. FNA-Tg, serum TSH, and serum Tg were higher in nonthyroidectomized patients than in thyroidectomized patients (P < .001, respectively). FNA-Tg was correlated with serum TSH and Tg levels (P < .001, respectively), and binary logistic regression analysis showed that serum TSH suppression and serum Tg presence independently affected the diagnosis made by FNA-Tg.

CONCLUSIONS Our results validated 1.0 ng/mL of FNA-Tg as a cutoff value for diagnosing LN metastasis of PTC and suggested that serum TSH suppression and serum Tg presence should be considered in diagnosing LN malignancy with FNA-Tg in PTC patients.