Antibodies to glutamic acid decarboxylase in the prediction of insulin dependency

Antibodies to glutamic acid decarboxylase (anti-GAD) predict the progression of adults masquerading as NIDDM to insulin dependency and predict the eventual occurrence of IDDM in healthy pregnant women in Finland. Almost 80% of prediabetic and newly diagnosed IDDM cases are positive for anti-GAD. However, approximately 20% of these groups do not have a humoral response to GAD so it cannot be claimed that anti-GAD is the exclusive autoimmune phenomenon. Nevertheless, 94% of children with newly diagnosed IDDM that we studied had an autoimmune response to either GAD, ICA or IAA, singly or in combination. The anti-GAD assay also has a substantial role in the diagnosis and classification of diabetes presenting in adult life since a proportion of adults who present with apparent NIDDM actually have a slowly evolving autoimmune insulitis, a condition we have called latent autoimmune diabetes in adults (LADA). It appears likely that anti-GAD will be predictive for IDDM in both first degree relatives and the general population. As a result of the cost and relative ease of performance, it will provide a practical alternative to ICA, particularly in population screening. Comparisons of testing for anti-GAD and ICA as predictors of IDDM using large population groups are now in progress in our laboratory.