C-peptide and autoimmune markers in diabetes.

Autoimmune markers such as islet cell antibodies (ICA), glutamic acid decarboxylase antibodies (GADA) and islet antigen-2 antibodies (IA-2A) are found in high frequencies among type 1 patients and especially among younger patients. Presence of these autoantibodies confirms the destructive process of the beta cells associated with immune-mediated type 1 diabetes. Type 2 diabetes is characterised by peripheral insulin resistance and a relative deficiency in insulin production. However, when autoimmune markers are analysed these are found in about 10% of patients clinically classified as type 2 diabetes, indicating that the frequency of type 1 diabetes is underestimated. GADA is the most frequent marker both among patients clinically classified as type 1 and type 2. GADA is also highly predictive for insulin treatment in patients not classified as type 1 diabetes. C-peptide is the best marker of the endogenous insulin production. Sampling of C-peptide is preferably done in the non-fasting condition since these values differentiate better between autoimmune and non-autoimmune diabetes. The presence of autoimmune markers at diagnosis predicts a course of further deteriorating beta cell function, whereas absence of autoimmune markers predicts stable beta cell function for the first two years in adults. Presence of GADA and in particular in high levels are prognostic for a low beta cell function within the next few years after diagnosis. Positivity only for ICA indicates a more preserved beta cell function for the first three years compared to positivity for other autoimmune markers.