Frequent Monitoring of C-peptide Levels in Newly Diagnosed Type 1 Subjects Using Dried Blood Spots Collected at Home

Objective: To evaluate a novel approach to measure beta-cell function by frequent testing of C-peptide concentrations in dried blood spots (DBS). Patients: Thirty-two children, aged 7-17 years, recently diagnosed with type 1 diabetes. Design: Mixed-meal-tolerance-test (MMTT) within 6 and again 12 months after diagnosis with paired venous and DBS C-peptide sampling at 0 and 90 minutes. Weekly DBS C-peptide before and after standardized breakfasts collected at home. Results: DBS and plasma C-peptide levels (n=115) correlated strongly (r=0.91; p<0.001). The Bland-Altman plot indicated good agreement. The median number of home-collected DBS cards per participant was 24 over a median of 6.9 months. Repeated DBS C-peptide levels varied considerably within and between subjects. Adjustment for corresponding home glucose measurements reduced the variance permitting accurate description of changes over time. The correlation of the C-peptide slope over time assessed by repeated home DBS versus area under the curve during the two MMTTs was r=0.73; p<0.001. Mixed models showed that a 1-month increase of diabetes duration was associated with 17 pmol/l decline in fasting DBS C-peptide, whereas increases of 1 mmol/l in glucose, 1 year older age-at-diagnosis and 100 pmol/l higher baseline plasma C-peptide were associated with 18, 17 and 61 pmol/l higher fasting DBS C-peptide levels, respectively. In addition, glucose responsiveness decreased with longer diabetes duration. Conclusion: Our approach permitted frequent assessment of C-peptide, making it feasible to monitor beta-cell function at home. Evaluation of changes in the slope of C-peptide using this method may permit short-term evaluation of promising interventions.