[Diagnosis of lymph node metastases from papillary thyroid carcinoma by measuring thyroglobulin in the puncture needle. Calculation of optimal cut-off point in our series].

Fine-needle aspiration cytology (FNAC), used in suspicious cervical lymph nodes of unknown origin is frequently inconclusive and prone to false negatives. In order to evaluate the usefulness of measuring thyroglobulin in the washing with saline solution of the puncture needle for the diagnosis of metastasis of papillary carcinoma of the thyroid, an optimal thyroglobulin cutting point has to be calculated, being positive or negative depending on whether the thyroglobulin levels are higher or lower than the cutting point. We have retrospectively studied 33 patients (19 women and 14 men) with an average age of 49.3 years, with papillary carcinoma of the thyroid and suspected lymph node metastasis. Of them 16 (47.1%) had a positive FNAC. To determine thyroglobulin predictive capacity with regards to the metastasis of papillary carcinoma of the thyroid a ROC analysis was carried out with an under curve area UCA: 0.987 (CI 95%: 0.808-1.000) obtaining, using Youden's J statistic, 0.4 ng/ml as the thyroglobulin cutting point with best predictive capacity. The study of the relationship between thyroglobulin and the preservation/non-preservation of the thyroid showed statistically significant differences (P=.023). Our results validate 0.4 ng/ml of thyroglobulin as an optimal cutting point of the presence of metastasis of papillary carcinoma of the thyroid in lymph nodes. When reviewing the bibliography, a great diversity of cutting points may be found, which is explained mainly by the great inter-observer and inter-assay variability. That is why we recommend calculating each laboratory's own optimal cutting point; and determine in subsequent studies two cutting points depending on whether or not thyroid is preserved.