Radioimmunoprecipitation assay for glutamic acid decarboxylase antibodies evaluated clinically with sera from patients with insulin-dependent diabetes mellitus.

We evaluated a new, commercially developed radioimmunoprecipitation assay for measuring glutamic acid decarboxylase (GAD) antibodies by using recombinant human GAD65. The intra- and interassay CVs were 8.0% (n = 20) and 8.6% (n = 15), respectively. We found GAD antibodies in 74% (23 of 31; 95% confidence interval 55-88%), 70% (14 of 20; 46-88%), and 65% (28 of 43; 49-79%) of patients at, respectively, < or = 1 year, 1-2 years, and 2-4 years after the onset of insulin-dependent diabetes mellitus (IDDM) and in 30% (30 of 99; 21-40%) of patients with long-term diabetes (4-22 years). We also detected GAD antibodies in 8% (9 of 106; 4-16%) of patients with non-insulin-dependent diabetes mellitus (NIDDM). The frequency of GAD antibodies in the NIDDM group was markedly higher in the insulin-deficient patients [67% (6 of 9; 30-93%)], who initially were nonketotic and non-insulin-dependent for > or = 6 months but later became insulin dependent, than in the non-insulin-deficient patients [3% (3 of 97; 1-9%)]. This new commercial assay is easy to use and provides a specific and sensitive method for evaluating GAD antibodies in IDDM.