A coated tube assay for the detection of blocking thyrotropin receptor autoantibodies

We developed a coated tube assay to discriminate TSH-receptor-stimulating autoantibodies [thyroid-stimulating antibodies (TSAb)] from those autoantibodies blocking TSH binding without intrinsic activation [thyroid-blocking antibodies (TBAb)]. The wild-type TSH receptor in the TSH binding-inhibitory assay was exchanged for a chimeric receptor where a TSAb epitope (amino acids 8-165) was replaced by comparable LH-R residues. Binding of (125)I-labeled TSH to this chimera could be inhibited by sera containing TBAb up to 95%. Sera from 316 patients with Graves' disease and 17 with autoimmune thyroid disease were grouped according to their bioassay activity. At the decision threshold, the chimera A assay had a sensitivity of 78.0% for TBAb with a specificity of 90.2%. In detail, 19 of 22 (86.4%) TBAb sera and 15 of 23 (65.2%) TSAb/TBAb sera were positive but only 32 of 216 (14.0%) TSAb sera and 5 of 72 (6.9%) bioassay negative sera. There was a weak but significant positive correlation (r = 0.46) between the chimera assay and the bioassay for TBAb. This is the first report of a coated tube assay for the determination of TBAb employing an adaptation of the TSH binding-inhibitory format, which could be a useful alternative to the bioassay.