Abstract
Background and Purpose Three network laboratories measured antibodies to islet autoantigens. Antibodies to glutamic acid decarboxylase (GAD65 [GADA]) and the intracellular portion of protein tyrosine phosphatase (IA-2ic [IA-2A]) were measured by similar, but not identical, methods in samples from participants in the Type 1 Diabetes Genetics Consortium (T1DGC).Methods All laboratories used radiobinding assays to detect antibodies to in vitro transcribed and translated antigen, but with different local standards, calibrated against the World Health Organization (WHO) reference reagent. Using a common method to calculate WHO units/mL, we compared results reported on samples included in the Diabetes Autoantibody Standardization Program (DASP), and developed standard methods for reporting in WHO units/mL. We evaluated intra-assay and inter-assay coefficient of variation (CV) in blind duplicate samples and assay comparability in four DASP workshops.Results Values were linearly related in the three laboratories for both GADA and IA-2A, and intra-assay technical errors for values within the standard curve were below 13% for GADA and below 8.5% for IA-2A. Correlations in samples tested 1–2 years apart were >97%. Over the course of the study, internal CVs were 10–20% with one exception, and the laboratories concordantly called samples GADA or IA-2A positive or negative in 96.7% and 99.6% of duplicates within the standard curve. Despite acceptable CVs and general concordance in ranking samples, the laboratories differed markedly in absolute values for GADA and IA-2A reported in WHO units/mL in DASP over a large range of values.Limitations With three laboratories using different assay methods (including calibrators), consistent values among them could not be attained.Conclusions Modifications in the assays are needed to improve comparability of results expressed as WHO units/mL across laboratories. It will be essential to retain high intra- and inter-assay precision, sensitivity and specificity and to confirm the accuracy of harmonized methods.
Highlights
The Type 1 Diabetes Genetics Consortium (T1DGC) comprises groups of investigators from many countries throughout the world, with a common goal of identifying genes predisposing to type 1 diabetes mellitus
The measurement was not used as an entry criterion for participation in the study, the research value of quantifying results in standardized World Health Organization (WHO) units/mL to allow more detailed phenotyping became apparent during the early stages of planning; i.e., that continuous values would permit additional analysis in relating genotypes to phenotypes
Given the international nature of the T1DGC and the extended distances that it covered, there was a clear need to establish regional laboratories, and three laboratories were selected on the basis of performance in Diabetes Autoantibody Standardization Program (DASP), a program organized by the Immunology of Diabetes Society and the Centers for Disease Control and Prevention
Summary
The Type 1 Diabetes Genetics Consortium (T1DGC) comprises groups of investigators from many countries throughout the world, with a common goal of identifying genes predisposing to type 1 diabetes mellitus. Three T1DGC network laboratories (in Asia-Pacific, Europe, and North America) were selected to measure antibodies to the islet autoantigens: glutamic acid decarboxylase (GAD65 [GADA]) and the intracellular portion of protein tyrosine phosphatase (IA-2ic [IA-2A]) as part of the determination of phenotypes for the project [1,2,3,4,5]. Antibodies to glutamic acid decarboxylase (GAD65 [GADA]) and the intracellular portion of protein tyrosine phosphatase (IA-2ic [IA-2A]) were measured by similar, but not identical, methods in samples from participants in the Type 1 Diabetes Genetics Consortium (T1DGC). Using a common method to calculate WHO units/mL, we compared results reported on samples included in the Diabetes Autoantibody Standardization Program (DASP), and developed standard methods for reporting in WHO units/mL. We evaluated intraassay and inter-assay coefficient of variation (CV) in blind duplicate samples and assay comparability in four DASP workshops
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