To explore the threshold and efficacy of thyroglobulin assessment in the needle washout after fine needle aspiration (FNA-Tg) in the preoperative diagnosis of cervical lymph metastasis of thyroid papillary carcinoma (PTC).

A retrospective analysis was performed on the FNA-Tg results of a total of 541 lymph nodes in 410 patients who underwent thyroid node surgery at the Department of Head Neck Surgery, Sir Run Run Shaw Hospital of Zhejiang University College of Medicine from July 2016 to January 2018. Taking the postoperative lymph node pathological results as the "gold standard", the ROC curve was plotted to obtain the optimal diagnostic threshold of FNA-Tg for cervical lymph node metastasis and evaluate its diagnostic efficacy. According to the level of serum thyroglobulin (sTg), the lymph nodes were divided into either a low sTg group (sTg ≤10 ng/ml) or a high sTg group (sTg >10 ng/ml), the ROC curve was plotted to analyze the diagnostic threshold of FNA-Tg in the two groups. The diagnostic threshold and efficacy of FNA-Tg/sTg ratio and FNA-Tg-sTg difference were then analyzed.

The ROC curve of FNA-Tg for diagnosis of lymph node metastasis was plotted, and the optimal diagnostic threshold of FNA-Tg was determined to be 0.835 ng/ml. Using this threshold as the cut-off point, the sensitivity and specificity were 88.5% and 97.5%, respectively, and the area under the ROC curve (AUC) was 0.944 (95%CI: 0.914~0.974). The level of FNA-Tg was correlated with the level of sTg (r=0.350, P＜0.001). Then, the lymph nodes were divided into either a low sTg group (n = 296) or a high sTg group (n = 245) according to the level of sTg, and the optimal diagnostic threshold for the two groups was determined to be 0.825 ng/ml (AUC = 0.909, 95%CI: 0.838~0.981) and 0.875 ng/ml (AUC = 0.951, 95%CI: 0.918~0.984), respectively. The optimal diagnostic cut-off of FNA-Tg/sTg ratio was determined to be 0.145; using this cut-off, the AUC of FNA-Tg/sTg ratio was 0.922 (95%CI: 0.889~0.955), and the sensitivity, specificity, positive predictive value, and negative predictive value were 82.7%, 95.5%, 85.3%, and 94.3%, respectively. The optimal diagnostic cut-off of FNA-Tg-sTg difference was determined to be >0; using this cut-off, the AUC of FNA-Tg-sTg difference was 0.841 (95%CI: 0.802~0.879), and the sensitivity, specificity, positive predictive value, and negative predictive value were 74.8%, 93.3%, 79.4%, and 91.5%, respectively.

FNA-Tg can be used for the preoperative diagnosis of cervical lymph node metastasis in patients with papillary thyroid carcinoma. When 0.835 ng/ml is used as the cut-off value of FNA-Tg, the diagnostic performance is better. To avoid the possible impact of sTg level, FNA-Tg/sTg ratio and FNA-Tg-sTg difference could be used in combination.