Appearance of islet cell autoantibodies after clinical diagnosis of diabetes mellitus

Islet cell antibodies (ICA) and glutamic acid decarboxylase antibodies (GAD 65Ab) are often present at diagnosis of insulin dependent diabetes mellitus (type I diabetes) and are supposed to decline in level and frequency durin g the first Sears of disease. We have analysed ICA and GAD65Ab at onset and after one year in 395 population based randomly selected 15-34 year old pa tients newly diagnosed with diabetes mellitus, to study how these autoantib odies persist, disappear and appear and their relation to C-peptide levels, Of the 395 samples 212 (54%) were positive for ICA, 250 (63%) were positive for GAD65Ab and 170 (43%) were positive for both, At follow up after one y ear, 27/183 (15%) of the ICA negative patients and 25/145 (17%) of the GAD6 5Ab negative patients had converted to positivity, Among the 103 patients n egative for both ICA and GAD65Ab, 16 turned positive for one or both antibo dies after one year. Patients converting to positivity for one or the other antibody after one g ear, had lower C-peptide levels after one year than patients who initially were and remained negative, supporting the hypothesis that these patients h ave a genuine type I diabetes. In conclusion, newly diagnosed patients may be negative for autoantibodies at diagnosis but develop these antibodies la ter on during the disease.