MEASUREMENT OF ANTIBODIES AGAINST GLUTAMIC-ACID-DECARBOXYLASE-65 (GADA) - 2 NEW I-125 ASSAYS COMPARED WITH [S-35] GAD 65-LIGAND BINDING ASSAY

Recently, 65-kDa glutamic acid decarboxylase (GAD 65) antibodies (GADA ) have been introduced as autoimmune markers in blood to confirm the d iagnosis of insulin-dependent diabetes mellitus (IDDM). In this study, to evaluate two new assays that use I-125-labeled GAD 65, we assayed samples from 100 children with recent onset of diabetes and 100 contro l children; the results were compared with those of a [S-35]GADA assay and with results for islet cell antibodies (ICA), the conventional au toimmune marker. Receiver operating characteristic (ROC) curve analysi s showed one of the new assays (from RSR) to be more sensitive (P = 0. 01) than the comparison ([S-35]GADA) assay, whereas the second new ass ay (from Elias) was less sensitive (P <0.001). The GADA frequency at 9 7.5% specificity was greatest in the comparison assay: 63 of 100 vs 41 of 100 (P < 0.01) and 53 of 100 (P = 0.16) in the RSR and Elias assay s, respectively. Almost all GADA-positive patients had ICA, but one-th ird of the ICA-positive patients was GADA-negative. Accordingly, addin g GADA analysis results to ICA testing increased the frequency of dete ction of autoimmune markers only slightly (from 81% to 85%). In conclu sion, at 97.5% specificity the [S-35]GADA assay seemed to be more effi cient than the I-125 assays, although the difference was significant o nly for the Elias I-125 assay. Antigen-specific antibodies other than GADA may explain the difference in GADA and ICA frequencies.