We investigated the diagnostic accuracy of a combined radioligand binding assay (RBA) for both glutamic acid decarboxylase (GAD) 65 and IA-2 autoantibodies (Ab), compared to separate RBAs for each autoantibody. 35S-GAD65 and 3H-IA-2 were produced by in vitro translation. Sera from 18 insulin-dependent diabetes mellitus (IDDM) patients and 20 healthy subjects were tested. Neither GAD65 nor IA-2 was detected by either combined or separate RBAs in the healthy subjects. In the IDDM patients, the concordance between the two RBAs for GAD65Ab was 100%. The concordance for IA-2, however, was 89%, because two sera with high titer GAD65Ab were falsely positive. There were strong correlations between the indexes of the two RBAs (r=0.982, r=0.965 for GAD65 and IA-2, respectively). In conclusion, the combined RBA for GAD65Ab and IA-2Ab has good diagnostic accuracy and will be a useful screening test for large numbers of NIDDM patients or for the general population.
