Endocrine Abstracts

Background: Blocking thyrotropin receptor (TSH-R) autoantibodies (TBAb) are present in 10-15% of patients with autoimmune thyroid disease (AITD). TBAb are functional and affect thyroid function. Analytical performance of a novel bioassay for TBAb was evaluated.

Methods: Serum samples from AITD patients were tested with a CE market cell-based TBAb blocking reporter bioassay (Thyretain®, Quidelortho, USA) with expression of a Luciferase transgene as readout and a new “Turbo^TM” TBAb bioassay (Quidelortho) with a readout that is based on a cyclic AMP-activated luciferase. All serum samples were also run on two TSH-R binding immunoassays (Cobas e411, Roche, Germany and ALINITY i Immunoassay-System, Abbott Germany). A Passing-Bablok regression, a Bland Altman plot, as well as user and lot comparisons were performed. Additionally, a dose-response curve was fitted for the TBAb Turbo and Thyretain bioassays via serial dilution and IC50 / IC80 values were compared.

Results: Of 1011 unselected, consecutive AITD patients, 131 patients (212 samples) were TBAb positive. Of 212 samples, 149 (70.3%), 47 (22%) and 16 (7.5%) were hypothyroid, euthyroid and hyperthyroid, respectively. The four TSH-R-Ab assays were negative in 90 control subjects devoid of autoimmune thyroid and endocrine disorders. In contrast, the Turbo^TM cAMP TBAb, Luciferase TBAb and the binding assays detected TBAb in 212 (100%), 168 (79%) and 138/180 (65%) samples, respectively (Chi-square test P < 0.001). The Turbo^TM TBAb bioassay highly correlated with thyroid function (Mann Whitney U test (MWU) P < 0.001). Furthermore, the magnitude of percentage inhibition in both Turbo^TM and Luciferase TBAb bioassays correlated with TSH-R-Ab binding assay positivity (both MWU P < 0.001). The two TBAb bioassays correlated (Spearman’s r = 0.8, p < 0.001) and the Bland-Altman plot displayed no significant bias (0.24). Values scatter with slight systemic deviation between TBAb mean values of 10–50% Inhibition with higher Turbo^TM TBAb than Luciferase TBAb results. Intra-assay validation demonstrated adequate precision with very low coefficient of variation (CoV) (average CoV 0.054) and lower CoV values with samples with high inhibitory effect (CoV_Average= 0.017 for a sample with 95% Inhibition Luciferase TBAb). CoV did not differ between users (p_CoV=0.35) and lots (p_CoV=0.121). IC50 / IC80 values were 1.7 ng/ml / 0.76 ng/ml and 6.76 ng/ml / 2.48 ng/ml) for Turbo and Luciferase TBAb bioassays demonstrating the markedly higher sensitivity of Turbo^TM.

Conclusions: The novel, easy-to-perform, rapid and reliable Turbo^TM TSH-R blocking bioassay detected significantly more TBAb than the established immunoassays emphasizing its higher analytical performance and clinical utility in the management of patients with AITD.