EP74 – Validation of a new competitive thyroglobulin assay in differentiated thyroid cancer patients with structural disease and comparison with immunometric assay and LC-MS/MS results

      Guastapaglia, Leila1; Kasamatsu, Teresa1; Nakabashi, Claudia Cristina Doimo1; Camacho, Cleber Pinto1,2; Maciel, Rui M. B.1,3; Vieira, Jose Gilberto1,3; Biscolla, Rosa Paula Mello1,3. 1 Division of Endocrinology, Universidade Federal de São Paulo, São Paulo, Brazil 2 Division of Endocrinology, UniNove São Paulo, São Paulo, Brazil 3 Division of Endocrinology, Grupo Fleury, São Paulo, Brazil   Background: Some pitfalls persist for thyroglobulin (Tg) measurement, in the follow up of patients with differentiated thyroid cancer (DTC): presence of thyroglobulin autoantibodies (TgAbs), and production of anomalous Tg. In these situations, low/undetectable Tg levels, measured by routine immunoassays (IMA), can be observed in patients with structural disease. Tg measurement by LC-MS/MS has been proposed as an alternative assay(1-4). Objective: To validate a new, in house, competitive Tg assay to be used in the follow up of metastatic DTC patients and compare the results with those by IMA and by LC-MS/MS. Methods: 21 metastatic patients:5 patients had IMA Tg levels compatible to the extent of the disease, and the remaining 16 had undetectable/ low IMA Tg levels. TgAbs were negative in 14 patients. The competitive assay employs a polyclonal antibody produced in rabbits immunized with human Tg, Tg labeled with biotin and for the solid phase separation, a monoclonal anti-rabbit IgG antibody adsorbed to microtiter plates. Functional sentitivity: 5.0 ng/mL. Results: In 5 patients, there was a concordance between IMA and competitive Tg assay,and the median level was 272 ng/mL.In 16 patients,despite of having structural disease,the patients presented not only low/undetectable Tg levels by IMA (<0.1- 5.1 ng/mL) but also by LC-MS/MS (0.5 – 18 ng/mL). All patients had Tg by the new assay higher and compatible with the metastatic disease (median of 90 ng/mL, range 8.2 – 400 ng/mL). DISCUSSION/CONCLUSION: The new competitive assay was able to detect Tg in all patients, independent of TgAbs presence, and presented a better performance than IMA and LC-MS/MS measurements.   References:
    1. Haugen BR, Alexander EK, Bible KC, Doherty GM, Mandel SJ, Nikiforov YE, et al. 2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer: The American Thyroid Association Guidelines Task Force on Thyroid Nodules and Differentiated Thyroid Cancer. Thyroid. 2016;26(1):1-133.
    2. Schlumberger M, Hitzel A, Toubert ME, Corone C, Troalen F, Schlageter MH, et al. Comparison of seven serum thyroglobulin assays in the follow-up of papillary and follicular thyroid cancer patients. J Clin Endocrinol Metab. 2007;92(7):2487-95.
    3. Kushnir, M.M., et al., Measurement of thyroglobulin by liquid chromatography-tandem mass spectrometry in serum and plasma in the presence of antithyroglobulin autoantibodies. Clin Chem, 2013. 59(6): 982-90.
    4. Netzel, B.C., et al., Thyroglobulin (Tg) Testing Revisited: Tg Assays, TgAb Assays, and Correlation of Results With Clinical Outcomes. J Clin Endocrinol Metab, 2015. 100(8): E1074-83.


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