Thyroglobulin Level in Fine-Needle Aspirates for Preoperative Diagnosis of Cervical Lymph Node Metastasis in Patients with Papillary Thyroid Carcinoma: Two Different Cutoff Values According to Serum Thyroglobulin Level

Background: Measurement of thyroglobulin (Tg) in the washout fluid of fine-needle aspirates (FNA-Tg) is useful for diagnosis of lymph node (LN) metastasis in papillary thyroid carcinoma (PTC). However, the cutoff value of FNA-Tg in the preoperative state is not defined clearly. This study aimed to evaluate the optimal cutoff value of preoperative FNA-Tg according to serum Tg level. Methods: FNA-Tg was measured in 135 PTC patients (160 LNs) for preoperative diagnosis of cervical LN metastasis. Results: Of the 160 LNs, 119 (74%) were surgically removed and 110 (69%) were diagnosed as malignant. When we adopted a FNA-Tg of 1.0 mu g/L as the cutoff value, the sensitivity and specificity were 99% and 76%, respectively. FNA-Tg levels were correlated with serum Tg levels (Pearson's coefficient 0.42, p=0.002) and the FNA-Tg levels of 12 of the 50 benign LNs were above 1.0 mu g/L. We classified the LNs into two groups according to serum Tg level regardless of anti-Tg antibody status: a low Tg group (<= 1.0 mu g/L, n=22, 14%) and a high Tg group (>1.0 mu g/L, n=138, 86%). In the low Tg group, the sensitivity and specificity of the FNA-Tg cutoff value of 1.0 mu g/L were 93% and 100%, respectively. In the high Tg group, the sensitivity and specificity of the FNA-Tg cutoff value of 19.0 mu g/L were 93% and 100%, respectively. A Tg ratio (FNA-Tg level divided by serum Tg level) of 0.5 gave an improved diagnostic performance (sensitivity, 98%; specificity, 98%) in the high Tg group. Conclusions: FNA-Tg levels in the preoperative state are affected by serum Tg levels when they exceeded 1.0 mu g/L. For the preoperative diagnosis of metastatic cervical LNs, it seems reasonable to employ different cutoff values of FNA-Tg depending on serum Tg levels. We propose the use of an optimal cutoff value of FNA-Tg of 1.0 mu g/L in patients with low serum Tg levels and a Tg ratio of 0.5 in those with high serum Tg levels irrespective of thyroglobulin antibody status.