Levels of C-peptide, body mass index and age, and their usefulness in classification of diabetes in relation to autoimmunity, in adults with newly diagnosed diabetes in Kronoberg, Sweden

OBJECTIVE: C-peptide is a main outcome measure in treatment trials of diabetes. C-peptide also has a role in the classification of diabetes, which is often difficult in adults and this is also increasingly recognised in adolescents and elders. AIM: We aimed to describe the levels of C-peptide in relation to age and body mass index (BMI) in a large population-based cohort of adults with newly diagnosed diabetes and compare the capabilities of C-peptide, age and BMI to discriminate between autoimmune and non-autoimmune diabetes. SUBJECTS AND METHODS: Blood samples from 1180 patients were analysed regarding islet cell antibody, glutamic acid decarboxylase antibody and fasting C-peptide (FCP). Receiver operating characteristics (ROC) curves were analysed to check the ability of age, BMI and C-peptide to discriminate between autoantibody-positive (Ab(+)) and -negative (Ab(−)) diabetes. RESULTS: Mean FCP was 0.73±0.5 (range 0.13–1.80) nmol/l in the Ab(+) and 1.42±0.9 (range 0.13–8.30) nmol/l in the Ab(−). FCP was 0.02 nmol/l higher per year increase in age at diagnosis of diabetes. Mean BMI was 26.0±4.8 (range 18.0–39.0) kg/m(2) in the Ab(+) and 28.9±5.3 (range 15.5–62.6) kg/m(2) in the Ab(−). FCP increased with age also within each BMI group. The highest area under the curve (AUC) in the ROC analysis was found for C-peptide, followed by age and BMI (0.78, 0.68 and 0.66 respectively). CONCLUSIONS: At diagnosis of diabetes, C-peptide was superior to age and BMI in discriminating between autoimmune and non-autoimmune diabetes. C-peptide increased significantly with BMI and age, latter also within each BMI group. Most of the adults had normal or high levels of C-peptide at presentation of diabetes among the autoimmune patients.