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Abstract Aim/Introduction To investigate the differences in the clinical significance and glutamic acid decarboxylase autoantibody (GADA) affinity between RIA (RIA‐GADA) and ELISA (ELISA‐GADA) in patients with type 1 diabetes. Methods A total of 415 patients with type 1 diabetes were enrolled, including 199 acute‐onset type 1 diabetes, 168 slowly progressive type 1 diabetes (SPIDDM), and 48 fulminant type 1 diabetes. GADA affinity was measured by a competitive binding experiment using unlabeled recombinant human GAD65 protein, and the diagnostic performance of both assays and the relationship between GADA affinity and the decline of fasting C‐peptide (F‐CPR) were examined. Results While the ELISA‐GADA displayed a higher sensitivity than the RIA method in diagnosing type 1 diabetes in acute‐onset patients, about 40% of SPIDDM patients with low‐titer RIA‐GADA were determined as negative by the ELISA method. Patients with type 1 diabetes with RIA‐GADA alone had an older age of onset, less diabetic ketoacidosis, a higher BMI, and a higher F‐CPR compared with patients positive for both RIA‐GADA and ELISA‐GADA. Additionally, 36% of RIA‐GADA‐positive patients had low‐affinity GADA (

Abstract Aims/Introduction This study aimed to investigate the clinical significance and antigen specificity of autoantibodies to insulinoma‐associated antigen‐2 (IA‐2A) by radioimmunoassay (RIA; IA‐2A‐RIA) and enzyme‐linked immunosorbent assay (ELISA; IA‐2A‐ELISA) in Japanese patients with type 1 diabetes. Materials and Methods A total of 338 type 1 diabetic patients were enrolled, including 38 fulminant type 1 diabetes, 168 acute‐onset type 1 diabetes and 137 slowly‐progressive type 1 diabetes (SPIDDM). The concordance, correlation of autoantibody titer, and the relationship between IA‐2A and progression to the insulin‐deficient state were examined. Also, competitive assay was used to examine the antigen specificity. Results The prevalence of IA‐2A‐ELISA was 4–5% lower than that of IA‐2A‐RIA in both the acute‐onset type 1 diabetes and SPIDDM, but the diagnostic sensitivities of both subtypes, when measured in combination with glutamic acid decarboxylase autoantibody, were comparable. The diagnosis of type 1 diabetes using either the RIA or ELISA methods showed substantial agreement with the exponential correlation of autoantibody titers detected by RIA and ELISA. Among the SPIDDM patients, the fasting C‐peptide for IA‐2A‐positive cases by ELISA, but not the RIA method, was significantly lower than in the negative cases (P