Study on cutoff value setting for differential diagnosis between Graves' disease and painless thyroiditis using the TRAb (Elecsys TRAb) measurement via the fully automated electrochemiluminescence immunoassay system.

The purposes of this study are to set the Elecsys TRAb cutoff value by which GD and PT can be accurately diagnosed simply; and to investigate the usefulness of the vascularity index (VI) obtained from power Doppler sonography (PDS). Using 109 normal controls, 186 GD patients, and 109 PT patients who were diagnosed through Tc-99m uptake, we set the cutoff value by conducting ROC analysis on the Elecsys TRAb values. The cutoff value as a result of the ROC analysis on the Elecsys TRAb values of the normal controls and GD patients was 0.8 IU/L with 100% of sensitivity and specificity. Because all 89 cases (81.6% of the entire PT cases) with Elecsys TRAb =<0.8 IU/L are PT, the cutoff =<0.8IU/L can thus be diagnosed as PT. In contrast, because all 166 cases (88.7% of the entire GD) with Elecsys TRAb >=3.0 IU/L except for one case of PT are GD, the cutoff >=3.0 IU/L can be diagnosed as GD. So Elecsys TRAb between 0.8-3.0 IU/L was dubbed gray zone (GZ). Finally, the cutoff value of 1.5 IU/L from the ROC on the PT and GD cases was chosen as the cutoff with 96.2% of sensitivity and 94.6 of specificity. All PDS VI >=80% were GD including 4 of 6 cases with GZ and all PDS VI <50% plus Elecsys TRAb-negative cases were PT including 4 of 5 cases with GZ. In conclusion, Elecsys TRAb cutoff and VI value for differential diagnosis between GD and PT has been set successfully.