Validation of serum hypersensitive thyroglobulin assay in alternative fine needle aspiration matrix.

[INTRODUCTION] Thyroglobulin levels in washout fluids from fine needle aspiration (FNA) biopsies of suspicious thyroid lesions is a well-established tool in diagnosis and follow-up of both papillary and follicular thyroid cancer. Despite the importance of this procedure, there is no standardization of the analytical process. We sought to validate an ultrasensitive thyroglobulin immunoassay not formulated for FNA washout fluids and to assess thyroglobulin stability based on matrix, storage temperature, and elapsed time before analysis.

[METHODS] Repeatability, limit of blank, limit of detection, and limit of quantification of thyroglobulin in 3 different matrices (kit diluent, serum free, and saline) were determined. Each sample was diluted and assayed immediately or after 14 or 21 days at ‒20 °C.

[RESULTS] Low coefficients of variation (s/x¯) × 100 were obtained, while limit-of-detection values for kit diluent and serum-free matrices resulted in values comparable to those reported for serum. Similar values of thyroglobulin were obtained in whole samples tested immediately or after 14 or 21 days of storage.

[CONCLUSIONS] Analytical validation of the ultrasensitive immunoassay for FNA samples was obtained for 3 matrices; matrix features and sample storage at ‒20 °C do not affect the reliability of thyroglobulin measurements. Use of a protein matrix is recommended for possible presence of samples with low analyte values.