Comparing the clinical significance and antigen specificity of insulinoma‐associated antigen‐2 autoantibodies between radioimmunoassay and enzyme‐linked immunosorbent assay in Japanese patients with type 1 diabetes

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