title: Measurement of islet cell antibodies in the Type 1 Diabetes Genetics Consortium: efforts to harmonize procedures among the laboratories.
abstract: BACKGROUND
  and
  
  
  OBJECTIVE
  Three network laboratories measured antibodies to islet autoantigens. Antibodies to glutamic acid decarboxylase (GAD65 [GADA]) and the intracellular portion of protein tyrosine phosphatase (IA-2(ic) [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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Clinical Science at North Bristol, University of Bristol, Bristol, UK.
authors.forename: Polly J
authors.initials: PJ
authors.lastname: Bingley
authors.forename: Alistair J K
authors.initials: AJ
authors.lastname: Williams
authors.forename: Peter G
authors.initials: PG
authors.lastname: Colman
authors.forename: Shane A
authors.initials: SA
authors.lastname: Gellert
authors.forename: George
authors.initials: G
authors.lastname: Eisenbarth
authors.forename: Liping
authors.initials: L
authors.lastname: Yu
authors.forename: Letitia H
authors.initials: LH
authors.lastname: Perdue
authors.forename: June J
authors.initials: JJ
authors.lastname: Pierce
authors.forename: Joan E
authors.initials: JE
authors.lastname: Hilner
authors.forename: Concepcion
authors.initials: C
authors.lastname: Nierras
authors.forename: Beena
authors.initials: B
authors.lastname: Akolkar
authors.forename: Michael W
authors.initials: MW
authors.lastname: Steffes
mesh_terms: D000906:Antibodies / Q000276:immunology*; D001324:Autoantigens / Q000032:analysis* / Q000276:immunology; D019411:Clinical Laboratory Techniques / Q000295:instrumentation* / Q000592:standards; D003625:Data Collection / Q000379:methods*; D003922:Diabetes Mellitus, Type 1 / Q000235:genetics*; D004493:Education; D014943:Global Health; D006801:Humans; D038622:Internationality; D007515:Islets of Langerhans / Q000276:immunology*; D011786:Quality Control; D012372:ROC Curve; D013223:Statistics as Topic
chemical_list: D000906:Antibodies; D001324:Autoantigens