Aims/hypothesis: To assess the prevalence of islet autoimmunity in adult-onset diabetes mellitus and the predictive value of islet autoantibodies in the general adult population of northern Italy.
Methods: A sample of 2076 people aged 40 years or more participating in the population-based Cremona Study and classified in 1990 as having diabetes mellitus, impaired and normal glucose tolerance according to WHO criteria after an oral glucose tolerance test, were tested for antibodies to glutamic acid decarboxylase and IA-2.
Results: Increased concentrations of glutamic acid decarboxylase antibodies were found in 4 (2.8 %) of 143 participants with known diabetes and none of 50 with previously unknown diabetes, 1 (0.65%) of 153 with impaired and 18 (1.0%) of 1718 with normal glucose tolerance. The increased prevalence of these antibodies in subjects with known diabetes was not statistically significant. Protein tyrosine phosphatase IA-2-antibodies were found in only four subjects, two of whom also had glutamic acid decarboxylase antibodies, all with normal glucose tolerance. After 8 years of follow-up, none of 21 non-diabetic subjects with either glutamic acid decarboxylase or IA-2-antibodies had developed diabetes and only a slight deterioration from normal to impaired fasting glucose was observed in 3 of 15 subjects with previous normal glucose tolerance.
Conclusion/interpretation: This study has shown that in northern Italy the prevalence of adult autoimmune diabetes in the general adult population is 0.19% (95 % CI 0.05-0.5); that autoimmune diabetes represents only a minority of all cases of adult diabetes; and that islet autoantibodies are not a high-risk factor for diabetes development in adults with normal glucose tolerance over 8 years of follow-up.