Background and aims: Autoantibodies to glutamate decarboxylase (GADA) are sensitive markers of type 1 diabetes (T1D). A comparison of radiobinding assays (RBAs) using either radiolabelled full-length human GAD65(1-585) or GAD65(96-585) in the 2012 islet autoantibody standardization program (IASP) workshop suggested that use of the N-terminally truncated GAD improved assay specificity. We aimed to determine whether a GAD65(96-585) based RBA identified GADA positive first-degree relatives of patients with T1D (FDRs) at increased risk of diabetes.
Subjects and methods: The first available sample from 280 FDRs participating in the Bart’s-Oxford family study of T1D previously found GADA positive with a local RBA that used GAD65(1-585) were re-assayed with the harmonized standard GADA assay using 35S-methionine labelled GAD65(1-585) or GAD65(96-585). Median age at first sample was 31 years (range 1 to 57 years) and median follow-up from first sample was 13.2 years (range 0.2 to 27 years). HLA class II genotyping was available on 208. Diabetes was ascertained by annual questionnaire. Assay thresholds were set at the 97.5th percentile of 222 healthy schoolchildren.
Results: Of 280 FDRs previously found positive for GADA, 256 were positive on re-assay using 35S-GAD65(1-585), 204 with 35S-GAD65(96-585) and 193 with both labels. Of 67 FDRs who progressed to diabetes, 63 were positive for GADA(1-585) and 61 positive for GADA(96-585). Kaplan-Meier survival analysis showed that because of their higher specificity, GADA(96-585) stratified risk of progression to diabetes in FDRs positive for GADA(1-585) (Figure 1, p<0.001). Of 13 FDRs carrying protective HLA-DQ6 haplotypes, 12 were positive for GADA(1-585), but only 3 were positive for GADA(96-585) (p=0.001).
Conclusion: Assays using N-terminally truncated GAD65(96-585) offer improved specificity for diabetes with minimal loss of sensitivity. The closer association of autoantibodies to GAD65(96-585) with diabetes risk suggests that GADA assays using N-terminally truncated GAD should be used for population screening to select participants for intervention trials.