We recently received the following question from a colleague: One of my endocrine docs posed the following question to me. Would you help me with an answer please?
“… a family has – for reasons we don’t need to get into – had their diabetes associated antibodies assessed at more than one lab. They have spoken with national experts on the meaning of these antibody results – and educated me to some degree. They advised me that there is a difference between a GAD Ab assay and a GAD-65 antibody assay (that the first is an umbrella term which includes the other but also assesses “non-65” GAD Ab’s, and that these “non-65 Ab’s” are not necessarily associated with type 1 diabetes).
Is this explanation true to your knowledge?” So, what do you think?
Response: In type 1 diabetes, autoreactivity to GAD65 is more common than autoreactivity to GAD67 (1). Therefore all assays, as far as we know, use GAD65 as the autoantigen in the immunoprecipitation assay even if the assay is simply termed a "GAD autoantibody" (GADA) assay. For example at the University of Florida, UF Pathology Laboratory, Endocrine Autoantibody Laboratory, we use the Kronus kit which tests for autoantibodies to GAD65 (and not GAD67) but we call our assay a GAD autoantibody (GADA) assay.
The family might be confused between GADA and islet cell cytoplasmic autoantibodies (ICA). ICA are determined by indirect immunofluorescence using human blood group O pancreas as substrate. There is at least one commercial kit for ICA; however, this kit uses primate pancreas as subtrate for ICA testing and not human pancreas. In the one published study comparing human and primate pancreas, use of the primate pancreas produced 6% false negatives and 5% false positives in comparison to the human pancreas (2). The authors of this study concluded: "It is therefore suggested that human pancreas should still be preferentially used for ICA determination, but baboon tissue may be a valuable substitute." Our editorial comment is that human pancreas remains the gold standard substrate for the ICA assay.
ICA can bind to several islet autoantigens including GAD, IA-2 (a protein tyrosine phosphatase) (3) and a glycolipid conjugate (4). A recent review concerning islet autoantibodies was published in 2010 (5).
- Velloso LA, Kämpe O, Hallberg A, Christmanson L, Betsholtz C, Karlsson FA. Demonstration of GAD-65 as the main immunogenic isoform of glutamate decarboxylase in type 1 diabetes and determination of autoantibodies using a radioligand produced by eukaryotic expression. J Clin Invest. 1993 May;91(5):2084-90.
- Scherbaum WA, Trischler G, Pfeiffer EF. Non-human primate pancreas as a substrate for the detection of islet-cell antibodies in human sera. Diabetes Res Clin Pract. 1989 Jun 20;7(1):1-5.
- Bergerot I, Souchier C, Thivolet C. Heterogeneity of islet cell cytoplasmic antibodies and autoantibodies to glutamate decarboxylase in relatives of patients with type 1 (insulin-dependent) diabetes. C R Acad Sci III. 1993 Nov;316(11):1368-73.
- Colman PG, Nayak RC, Campbell IL, Eisenbarth GS. Binding of cytoplasmic islet cell antibodies is blocked by human pancreatic glycolipid extracts Diabetes. 1988 May;37(5):645-52.
- Winter WE, Schatz DA. Autoimmune markers in diabetes. Clin Chem. 2011 Feb;57(2):168-75.