Thyroglobulin measurement in fine-needle aspirate washouts: the criteria for neck node dissection for patients with thyroid cancer

Several studies report that detection of thyroglobulin (Tg) in fine-needle aspiration (FNA) biopsy washout fluid from lymph nodes identifies recurrences/metastases of differentiated papillary thyroid cancer (DPTC) in the neck with higher sensitivity and specificity than fine-needle aspiration cytology (FNAC). However, the diagnostic FNA-Tg cutoff values have not yet been established. To determine an appropriate diagnostic threshold value for Tg levels in FNA washout fluid in patients with neck node metastases or recurrences of DPTC. We performed ultrasound (US)-guided FNAC and measured Tg levels in FNA washout fluid (FNA-Tg). Final diagnoses were confirmed by histological examination of excised specimens or by follow-up examinations for at least 24 months. A total of 168 ultrasonographically detected lymph nodes from 168 patients with DPTC were included. In comparison with FNAC, we evaluated diagnostic sensitivity, specificity and accuracy of metastasis detection according to several predetermined threshold levels: 1, 10, 100 ng/ml, mean+2SD of node-negative patients, and FNA-Tg/serum-Tg > 1. The diagnostic sensitivity was lowest at 77.3% for FNAC alone. The lower FNA-Tg threshold levels showed higher diagnostic sensitivity whereas the higher FNA-Tg showed higher specificity. In addition to the FNAC results, FNA-Tg levels showed 95.0% sensitivity, 81.6% specificity, 92.6% in positive predictive value (PPV) and 87.0% in negative predictive value (NPV) with the threshold of FNA-Tg level at 10 ng/ml, serum-Tg or mean+2SD of FNA-Tg measured in node-negative patients. The diagnostic accuracy of FNA-Tg was higher in neck node recurrences after thyroid surgery than in the metastasis of patients waiting for surgery. FNA-Tg measurement as well as FNAC should be performed either before or after surgery to evaluate neck node metastases or recurrences in patients with differentiated thyroid carcinomas. We recommend that the threshold values for FNA-Tg levels should be > 10 ng/ml if the serum-Tg level or the mean+2SD in node-negative patients is not available for reference.