Autoantibody Assays for National Institute of Diabetes and Digestive and Kidney Harmonization of Glutamic Acid Decarboxylase and Islet Antigen-2

Background/Rationale: Autoantibodies to islet antigen-2 (IA-2A) and glutamic acid decarboxylase (GADA) are markers for diagnosis, screening, and measuring outcomes in National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) consortia studies. A harmonization program was established to increase comparability of results within and among these studies. Methods: Large volumes of six working calibrators were prepared from pooled sera with GADA 4.8 – 493 World Health Organization (WHO) units/ml and IA-2A 2–235 WHO units/ml. Harmonized assay protocols for IA-2A and GADA using 35 S-methionine-labelled in vitro transcribed and translated antigens were developed based on methods in use in three NIDDK laboratories. Antibody thresholds were defined using sera from patients with recent onset type 1 diabetes and healthy controls. To evaluate the impact of the harmonized assay protocol on concordance of IA-2A and GADA results, two laboratories retested stored TEDDY study sera using the harmonized assays. Results: The harmonized assays gave comparable but not identical results in the three laboratories. For IA-2A, using a common threshold of 5 DK units/ml, 549 of 550 control and patient samples were concordantly scored as positive or negative, specificity was greater than 99% with sensitivity 64% in all laboratories. For GADA, using thresholds equivalent to the 97th percentile of 974 control samples in each laboratory, 1051 (97.9%) of 1074 samples were concordant. On the retested TEDDY samples, discordance decreased from 4 to 1.8% for IA-2A (n ϭ 604 samples; P ϭ 0.02) and from 15.4 to 2.7% for GADA (n ϭ 515 samples; P Ͻ 0.0001). Conclusion: Harmonization of GADA and IA-2A is feasible using large volume working calibrators and common protocols and is an effective approach to ensure consistency in autoantibody measurements. T he measurement of islet autoantibodies is used extensively in diabetes research to identify individuals at risk of developing type 1 diabetes, in particular as selection criteria for clinical prevention trials. It is also increasingly used in the classification of diabetes (1). Such activities often require multicenter recruitment with islet autoanti-body screening carried out in central laboratories. There has been substantial progress toward standardization of glutamic acid decarboxylase (GAD) and islet antigen-2 (IA-2) antibodies through the Diabetes Autoantibody Standardization Program (DASP), a collaboration between the Immunology of Diabetes Society and Centers for Disease Control, and reliable assays and laboratories can be identified and new assays evaluated (2). Previous comparisons have, however, demonstrated that, despite high sensitivity and specificity and general concordance in …