title: Calcium addition to EDTA plasma eliminates falsely positive results in the RSR GADAb ELISA.
abstract: BACKGROUND
  The RSR ELISAs for GADAb and IA-2Ab are now widely used but due to a considerable number of falsely positive GADAb results obtained with EDTA plasma, it is recommended that only serum samples are run in these two assays. However, as EDTA plasma is often the preferred sample type in Europe we have assessed the possibility that adding Ca(2+) to plasma would reduce the number of falsely positive results.
  
  
  METHODS
  Antibody positive as well as antibody negative diabetic subjects were included in the study (n=80). Serum and EDTA plasma were collected from the same subjects at the same occasion. Samples were divided into three groups; sera, EDTA plasma and EDTA plasma+Ca(2+). ELISA kits were supplied by RSR Ltd and performed according to the manufacturers instructions.
  
  
  RESULTS
  GADAb analyses with plasma showed a markedly increased prevalence of GADAb positive subjects (81%) compared to sera (36%), indicating falsely positive results with plasma. Addition of Ca(2+) reduced the number of GADAb positive results to the same number obtained with serum. IA-2Ab positivity was not different between sera and plasma.
  
  
  CONCLUSIONS
  EDTA plasma can be used in the RSR GADAb and IA-2Ab ELISAs if Ca(2+) is added prior to assay.
authors.affiliation: Department of Clinical Sciences, Diabetes Research Laboratory, Lund University, Sweden.
authors.forename: Emma
authors.initials: E
authors.lastname: Nilson
authors.forename: Birgitte
authors.initials: B
authors.lastname: Ekholm
authors.forename: Bernard
authors.initials: B
authors.lastname: Rees Smith
authors.forename: Carina
authors.initials: C
authors.lastname: Törn
authors.forename: Magnus
authors.initials: M
authors.lastname: Hillman
mesh_terms: D001323:Autoantibodies / Q000097:blood* / Q000276:immunology*; D002118:Calcium / Q000494:pharmacology*; D004492:Edetic Acid / Q000097:blood*; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods*; D005189:False Positive Reactions; D005968:Glutamate Decarboxylase / Q000276:immunology* / Q000378:metabolism; D006801:Humans; D010949:Plasma / Q000187:drug effects / Q000378:metabolism*
chemical_list: D001323:Autoantibodies; D004492:Edetic Acid; D005968:Glutamate Decarboxylase; D002118:Calcium