A NOVEL RADIOLIGAND BINDING ASSAY TO DETERMINE DIAGNOSTIC-ACCURACY OFISOFORM-SPECIFIC GLUTAMIC-ACID DECARBOXYLASE ANTIBODIES IN CHILDHOOD IDDM

Insulin-dependent diabetes mellitus (IDDM) is associated with autoreac tivity against GAD but the diagnostic sensitivity (positivity in disea se) and specificity (negativity in health) of isoform-specific GAD ant ibodies have yet to be defined in assay systems suitable for screening large number of samples. One set of IDDM patient (n = 10) and control (n = 50) standard sera were used to develop quantitative antibody ass ays with in vitro synthesized recombinant S-35-methionine-labelled GAD 65 and GAD67, respectively, and protein A-Sepharose to separate free f rom antibody-bound ligand. Binding levels were not normally distribute d (p < 0.0001) and therefore, the diagnostic accuracy of GAD antibodie s was analysed by the ROC plots in population-based, consecutively-dia gnosed, matched, healthy control subjects (n = 157). The ROC plots sho wed that the diagnostic sensitivity of GAD65 antibodies was 77 % and t he specificity 92 % compared with 8 % and 98 %, respectively for GAD67 antibodies. In the IDDM sera, GAD65 and GAD67 antibodies were concord ant in 7 % (6 of 81) and GAD65 antibodies and ICA in 89 % (72 of 81) w ithout a correlation between the autoantibody levels. Autoantibodies t o recombinant human islet GAD65 are specific and sensitive markers for childhood IDDM in this immunoassay with in vitro synthesized S-35-met hionine-labelled recombinant GAD.