The first 142 amino acids of glutamate decarboxylase do not contribute to epitopes recognized by autoantibodies associated with Type 1 diabetes

Aims Glutamate decarboxylase ( GAD ) antibodies are the most widely used predictive marker for Type 1 diabetes, but many individuals currently found to be GAD antibody‐positive are unlikely to develop diabetes. We have shown previously that radioimmunoassays using N‐terminally truncated 35 S‐ GAD 65 (96–585) offer better disease specificity with similar sensitivity to full‐length 35 S‐ GAD 65 (1–585). To determine whether assay performance could be improved further, we evaluated a more radically truncated 35 S‐ GAD 65 (143–585) radiolabel. Methods Samples from people with recent‐onset Type 1 diabetes ( n = 157) and their first‐degree relatives ( n = 745) from the Bart's–Oxford family study of childhood diabetes were measured for GAD antibodies using 35 S‐labelled GAD 65 (143–585). These were screened previously using a local radioimmunoassay with 35 S‐ GAD 65 (1–585). A subset was also tested by enzyme‐linked immunosorbent assay ( ELISA ), which performs well in international workshops, but requires 10 times more serum. Results were compared with GAD antibody measurements using 35 S‐ GAD 65 (1–585) and 35 S‐ GAD 65 (96–585). Results Sensitivity of GAD antibody measurement was maintained using 35 S‐ GAD 65 (143–585) compared with 35 S‐ GAD 65 (1–585) and 35 S‐ GAD 65 (96–585). Specificity for Type 1 diabetes was improved compared with 35 S‐ GAD 65 (1–585), but was similar to 35 S‐ GAD 65 (96–585). Relatives found to be GAD antibody‐positive using these truncated labels were at increased risk of diabetes progression within 15 years, compared with those positive for GAD (1–585) antibody only, and at similar risk to those found GAD antibody‐positive by ELISA . Conclusions The first 142 amino acids of GAD 65 do not contribute to epitopes recognized by Type 1 diabetes‐associated GAD antibodies. Low‐volume radioimmunoassays using N‐terminally truncated 35 S‐ GAD 65 are more specific than those using full‐length GAD 65 and offer practical alternatives to the GAD antibody ELISA for identifying children at increased risk of Type 1 diabetes.