Early Detection of β-Cell Decline Using Home Dried Blood Spot C-Peptide Levels in New-Onset Type 1 Diabetes.

OBJECTIVE: There is a need for early detection of β-cell decline in people with newly diagnosed (ND) type 1 diabetes. The gold standard mixed-meal tolerance test (MMTT) is an invasive and time-consuming procedure that requires participants to travel to clinical sites. We assessed the feasibility of measuring dried blood spot (DBS) C-peptide levels collected at home as an alternative to the MMTT to detect early β-cell decline. RESEARCH DESIGN AND METHODS: Individuals with ND type 1 diabetes were recruited within 6 weeks of diagnosis as part of the INNODIA cohort. They collected finger-prick DBS C-peptide data at home, both while fasting and 60 min after a liquid meal. At 12 months, an MMTT was conducted to measure the area under the curve (AUC) of venous C-peptide. RESULTS: Data of 292 people were analyzed (mean age 12.7 years; range 1.2-43.8 years). The median number of DBS card pairs per participant was 6.5 (interquartile range = 2, 9) over 12 months. The slopes of stimulated DBS C-peptide levels in the first 6 months significantly predicted venous MMTT AUC C-peptide and peak C-peptide levels at 12 months (P < 0.01). The models were adjusted for simultaneous glucose levels, age, and baseline fasting C-peptide level. However, the 6-month fasting DBS C-peptide slope did not predict 12-month MMTT AUC C-peptide. CONCLUSIONS: Home DBS C-peptide assessment is a feasible method to monitor β-cell function in ND type 1 diabetes. The early prognostic utility of stimulated DBS C-peptide highlights its potential for optimizing trial design. However, further validation is needed to confirm its reliability and broader applicability.