Recently a new procedure for measuring serum TSH receptor (TSHR) autoantibody (TRAb) was reported by Smith et al. in which the autoantibodies inhibit binding of a human monoclonal thyroid stimulating antibody M22 (labeled with biotin) to TSHR-coated ELISA plate wells (pTRAb(3rd) assay). The aim of this study was to compare the performance of pTRAb3rd assay with pTRAb(2nd) assay based on inhibition of TSH-biotin binding to TSHR-coated ELISA plate wells. In addition, we evaluated the applicability of TRAb(3rd) assay to discriminate between untreated Graves' disease (GD) and painless thyroiditis (PT). Analysis of sera from 230 healthy controls indicated that only 1 (0.43%) gave inhibition of M22-binding values of greater than 15% (32.8% inhibition). To define the clinical cut-off point for a positive serum with autoantibodies to the TSHR, we performed receiver operating characteristic curve of the data from 244 untreated GD and three different control groups for pTRAb(3rd) assay. With a sensitivity of 99.6% at a cut-off of 14.5%, 22.0% and 22.0% inhibition of M22 binding, the specificity of healthy controls without PT, with PT and with PT excluding postpartum PT and PT during remission of GD was 99.6%, 96.6% and 97.5%, respectively. The pTRAb(3rd) assay was closely correlated to pTRAb(2nd) assay in the 244 untreated Graves' sera (r = 0.911). The pTRAb(3rd) assay detected 243 of 244 (99.6%) untreated GD, whereas 9.2% of PT and 6.7% of the subacute thyroiditis (SAT) were detectable. In contrast, pTRAb(2nd) assay detected 242 of 244 (99.2%) Graves' same sera, while 16.8% from PT's same sera and 13.3% from SAT were detectable. In conclusion, pTRAb(3rd) assay has significantly (p = 0.0026) superior diagnostic accuracy for GD and PT, compared to that of pTRAb(2nd) assay.
