Bridge Technology with TSH Receptor Chimera for Sensitive Direct Detection of TSH Receptor Antibodies Causing Graves' Disease: Analytical and Clinical Evaluation

Graves' disease is caused by stimulating autoantibodies against the thyrotropin receptor inducing uncontrolled overproduction of thyroid hormones. A Bridge Assay is presented for direct detection of these thyroid-stimulating immunoglobulins using thyrotropin receptor chimeras. A capture receptor, formed by replacing aa residues 261-370 of the human thyrotropin receptor with residues 261-329 from rat lutropin/choriogonadotropin receptor and fixed to microtiter plates, binds one arm of the autoantibody. The second arm bridges to the signal receptor constructed from thyrotropin receptor (aa21-261) and secretory alkaline phosphatase (aa1-519) inducing chemiluminescence. The working range of the assay is from 0.3IU/l to 50IU/l with a cutoff of 0.54IU/l and functional sensitivity of 0.3IU/l. Sensitivity and specificity are 99.8 and 99.1%, respectively, with a diagnostic accuracy of 0.998. The low grey zone is from 0.3-0.54IU/l. The stimulatory character of the assayed antibodies is shown through a good correlation (r=0.7079, p<10(-7)) to serum T4 levels of untreated patients. In Graves' disease, titers are increased in associated eye disease. In 3 hypothyroid patients with sera positive in the thyrotropin receptor competition assay and in the blocking bioassay, antibodies are not detected by the Bridge Assay, while the monoclonal blocking antibody K1-70 was detected. In Hashimoto disease thyrotropin receptor autoantibodies are detected in some patients, but not in goiter. This Bridge Assay delivers good diagnostic accuracy for identification of Graves' disease patients. Its high sensitivity may facilitate early detection of onset, remission, or recurrence of Graves' disease enabling timely adaption of the treatment.