Evaluation of a novel radioimmunoassay using I-125-labelled human recombinant GAD65 for the determination of glutamic acid decarboxylase (GAD65) autoantibodies

Autoantibodies to the islet, cell 65-kilodalton isoform of glutamic acid de carboxylase (GAD65) are present in most patients with type 1 diabetes melli tus years before the clinical manifestation of the disease. GAD65 autoantib odies are also present in a subset of patients with type 2 diabetes who fre quently become insulin dependent. In the present study, we evaluated a new, commercially available radioimmunoprecipitation assay for measuring GAD65 autoantibodies using I-125-labelled human recombinant GAD65. Results obtain ed with this assay were compared with those obtained by a reference assay b ased on S-35-labelled recombinant GAD65. Analyses were performed on 67 pati ents with type I diabetes, 350 with type 2 diabetes, and 150 apparently hea lthy individuals. An excellent agreement was found between the results obta ined by the I-125-GAD65 assay and those obtained by the reference method. T he receiver operating characteristic (ROC) curve analysis was used to evalu ate the sensitivity and specificity of the two assays. The sensitivity of e ach assay was determined from the results of the 67 type 1 patients, while the specificity was based on the 150 healthy individuals. Based on the ROC curves, the two assays appeared identical, with a sensitivity of 84% and a clinical specificity of 98%. In conclusion, based on our results, this simp le, one-step centrifugation, high-capacity I-125-GAD65 assay has the same s ensitivity and specificity as the reference assay and is highly suitable to detect GAD65 autoantibodies in human samples.